Make it possible to build on modern Debian

Building xtreemfs on Debian 12 yields the following error:
```
[ERROR] Source option 6 is no longer supported. Use 7 or later.
[ERROR] Target option 6 is no longer supported. Use 7 or later.
```
Setting java.source.version and java.target.version to 11 in the pom.xml files
make it possible to build on a modern OS. It would also allow for
updating the dockerfiles in xtreemfs-docker repo to a modern OS.

Signed-off-by: Micke Nordin <kano@sunet.se>