The warning you get suggests you did you run the first command on https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationViaAPT/#HAPTConfiguration.
wget -q "https://maven.xwiki.org/public.gpg" -O- | sudo apt-key add -
XWiki recommend Java 8 (because it’s mostly tested with Java 8 right now) since a very long time now so the short answer is any version you want. The repository you used will install the very last version of XWiki which is probably what you want.