Resolve Java 21 FTBFS
This MR fixes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1053079
Changes:
- Use java_compat_level variable provided by java-common to adjust -source/-target level to the minimum required by the default Java.
 - Remove trailing whitespaces in the patch
 
Testing:
- build in sid chroot with default Java 21
 - build in sid chroot