Use compat level macro to resolve Java 21 FTFBS

java-common provides a new Makefile varaible java-compat-level that specifies the minimum supported language level. Use it instead of the hardcoded 1.7 value in the makefile.

Testing:

• rebuild in sid chroot with default Java 21 succeeds after the patch.

mentioned in merge request javamorph!2 (merged)

added 8 commits

• 86121396...b946c429 - 7 commits from branch java-team:master
• 8bbcb6ff - Merge branch 'master' into 'use-compat-level-macro'

Compare with previous version

mentioned in commit b8b5dcf6

merged