Fix Java 21 FTBFS
This MR addresses: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052580
Changes:
- Use JAVA_COMPAT_LEVEL environment variable provided by java-common to adjust -source/-target level to the minimum required by the default Java.
Testing:
- build in sid chroot with default Java 21
- build in sid chroot