There seems to be a problem with the Maven repository:
…
Get:5 http://maven.xwiki.org stable/ Release [1308 B]
Ign:6 http://maven.xwiki.org stable/ Release.gpg
…
Reading package lists… Done
E: The repository ‘http://maven.xwiki.org stable/ Release’ is no longer signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.