That fixed the issue! All of the errors in catalina.out are gone.
Steps taken:
wiki:/var/lib/xwiki/data/store$ sudo rm -rf solr
wiki:/var/lib/xwiki/data/cache$ sudo rm -rf solr
catalina.out:
[2023-04-06 17:34:32] [info] Server version name: Apache Tomcat/9.0.58 (Ubuntu)
[2023-04-06 17:34:32] [info] Server built: Jan 6 1970 15:09:28 UTC
[2023-04-06 17:34:32] [info] Server version number: 9.0.58.0
[2023-04-06 17:34:32] [info] OS Name: Linux
[2023-04-06 17:34:32] [info] OS Version: 5.15.0-69-generic
[2023-04-06 17:34:32] [info] Architecture: amd64
[2023-04-06 17:34:32] [info] Java Home: /usr/lib/jvm/java-11-openjdk-amd64
[2023-04-06 17:34:32] [info] JVM Version: 11.0.18+10-post-Ubuntu-0ubuntu122.04
[2023-04-06 17:34:32] [info] JVM Vendor: Ubuntu
[2023-04-06 17:34:32] [info] CATALINA_BASE: /var/lib/tomcat9
[2023-04-06 17:34:32] [info] CATALINA_HOME: /usr/share/tomcat9
[2023-04-06 17:34:33] [info] Command line argument: --add-opens=java.base/java.lang=ALL-UNNAMED
[2023-04-06 17:34:33] [info] Command line argument: --add-opens=java.base/java.io=ALL-UNNAMED
[2023-04-06 17:34:33] [info] Command line argument: --add-opens=java.base/java.util=ALL-UNNAMED
[2023-04-06 17:34:33] [info] Command line argument: --add-opens=java.base/java.util.concurrent=ALL-UNNAMED
[2023-04-06 17:34:33] [info] Command line argument: --add-opens=java.rmi/sun.rmi.transport=ALL-UNNAMED
[2023-04-06 17:34:33] [info] Command line argument: -Djava.util.logging.config.file=/var/lib/tomcat9/conf/logging.properties
[2023-04-06 17:34:33] [info] Command line argument: -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
[2023-04-06 17:34:33] [info] Command line argument: -Xms800m
[2023-04-06 17:34:33] [info] Command line argument: -Xmx1600m
[2023-04-06 17:34:33] [info] Command line argument: -Dfile.encoding=utf-8
[2023-04-06 17:34:33] [info] Command line argument: -Djava.awt.headless=true
[2023-04-06 17:34:33] [info] Command line argument: -XX:+UseParallelGC
[2023-04-06 17:34:33] [info] Command line argument: -XX:MaxGCPauseMillis=100
[2023-04-06 17:34:33] [info] Command line argument: -javaagent:/usr/share/java/glowroot/glowroot.jar
[2023-04-06 17:34:33] [info] Command line argument: -Dorg.apache.tomcat.util.buf.UDecoder.ALLOW_ENCODED_SLASH=true
[2023-04-06 17:34:33] [info] Command line argument: -Dorg.apache.catalina.connector.CoyoteAdapter.ALLOW_BACKSLASH=true
[2023-04-06 17:34:33] [info] Command line argument: -Djdk.tls.ephemeralDHKeySize=2048
[2023-04-06 17:34:33] [info] Command line argument: -Djava.protocol.handler.pkgs=org.apache.catalina.webresources
[2023-04-06 17:34:33] [info] Command line argument: -Dorg.apache.catalina.security.SecurityListener.UMASK=0027
[2023-04-06 17:34:33] [info] Command line argument: -Dignore.endorsed.dirs=
[2023-04-06 17:34:33] [info] Command line argument: -Dcatalina.base=/var/lib/tomcat9
[2023-04-06 17:34:33] [info] Command line argument: -Dcatalina.home=/usr/share/tomcat9
[2023-04-06 17:34:33] [info] Command line argument: -Djava.io.tmpdir=/tmp
[2023-04-06 17:34:33] [info] Loaded Apache Tomcat Native library [1.2.31] using APR version [1.7.0].
[2023-04-06 17:34:33] [info] APR capabilities: IPv6 [true], sendfile [true], accept filters [false], random [true], UDS [true].
[2023-04-06 17:34:33] [info] APR/OpenSSL configuration: useAprConnector [false], useOpenSSL [true]
[2023-04-06 17:34:33] [info] OpenSSL successfully initialized [OpenSSL 3.0.2 15 Mar 2022]
[2023-04-06 17:34:33] [info] Initializing ProtocolHandler ["http-nio-8080"]
[2023-04-06 17:34:33] [info] Server initialization in [1525] milliseconds
[2023-04-06 17:34:34] [info] Starting service [Catalina]
[2023-04-06 17:34:34] [info] Starting Servlet engine: [Apache Tomcat/9.0.58 (Ubuntu)]
[2023-04-06 17:34:34] [info] Deploying deployment descriptor [/etc/tomcat9/Catalina/localhost/xwiki.xml]
[2023-04-06 17:34:34] [warning] The path attribute with value [/xwiki] in deployment descriptor [/etc/tomcat9/Catalina/localhost/xwiki.xml] has been ignored
[2023-04-06 17:34:40] [info] 2023-04-06 17:34:40,962 [main] INFO iPropertiesConfigurationSource - Loading [xwiki.properties] from default location [/etc/xwiki/xwiki.properties]
[2023-04-06 17:34:41] [info] 2023-04-06 17:34:41,256 [main] INFO o.x.e.i.ServletEnvironment - Using permanent directory [/var/lib/xwiki/data]
[2023-04-06 17:35:00] [info] 2023-04-06 17:35:00,911 [main] INFO o.x.s.s.i.EmbeddedSolr - Generating a new Solr home directory at [/var/lib/xwiki/data/store/solr]
[2023-04-06 17:35:00] [info] 2023-04-06 17:35:00,925 [main] INFO o.x.s.s.i.EmbeddedSolr - Starting embedded Solr server...
[2023-04-06 17:35:00] [info] 2023-04-06 17:35:00,925 [main] INFO o.x.s.s.i.EmbeddedSolr - Using Solr home directory: [/var/lib/xwiki/data/store/solr]
[2023-04-06 17:35:02] [info] 2023-04-06 17:35:02,841 [main] WARN o.e.j.u.s.S.config - Trusting all certificates configured for Client@2d739a6f[provider=null,keyStore=null,trustStore=null]
[2023-04-06 17:35:02] [info] 2023-04-06 17:35:02,842 [main] WARN o.e.j.u.s.S.config - No Client EndPointIdentificationAlgorithm configured for Client@2d739a6f[provider=null,keyStore=null,trustStore=null]
[2023-04-06 17:35:03] [info] 2023-04-06 17:35:03,096 [main] WARN o.e.j.u.s.S.config - Trusting all certificates configured for Client@5ff24b0f[provider=null,keyStore=null,trustStore=null]
[2023-04-06 17:35:03] [info] 2023-04-06 17:35:03,096 [main] WARN o.e.j.u.s.S.config - No Client EndPointIdentificationAlgorithm configured for Client@5ff24b0f[provider=null,keyStore=null,trustStore=null]
[2023-04-06 17:35:03] [info] 2023-04-06 17:35:03,294 [main] WARN o.a.s.c.CoreContainer - Not all security plugins configured! authentication=disabled authorization=disabled. Solr is only as secure as you make it. Consider configuring authentication/authorization before exposing Solr to users internal or external. See https://s.apache.org/solrsecurity for more info
[2023-04-06 17:35:06] [info] 2023-04-06 17:35:06,879 [main] INFO o.x.s.s.i.EmbeddedSolr - Started embedded Solr server.
[2023-04-06 17:35:16] [info] 2023-04-06 17:35:16,158 [main] INFO o.x.s.f.i.FilesystemStoreTools - Using filesystem store directory [/data/xwiki/data/store/file]
[2023-04-06 17:35:16] [info] Deployment of deployment descriptor [/etc/tomcat9/Catalina/localhost/xwiki.xml] has finished in [42,426] ms
[2023-04-06 17:35:16] [info] Deploying web application directory [/var/lib/tomcat9/webapps/ROOT]
[2023-04-06 17:35:17] [info] At least one JAR was scanned for TLDs yet contained no TLDs. Enable debug logging for this logger for a complete list of JARs that were scanned but no TLDs were found in them. Skipping unneeded JARs during scanning can improve startup time and JSP compilation time.
[2023-04-06 17:35:17] [info] Deployment of web application directory [/var/lib/tomcat9/webapps/ROOT] has finished in [1,020] ms
[2023-04-06 17:35:17] [info] Starting ProtocolHandler ["http-nio-8080"]
[2023-04-06 17:35:17] [info] Server startup in [43670] milliseconds
Also and maybe coincidentally, the server started responding to requests immediately without the need for any restarts (see: Xwiki not responding to requests). Which is very exciting.