Resolve Java 21 ftbs

This MR resolves 1057521.

Changes:

• debian/patches/01-edit-gradle-buildsystem.patch: ignore 'serial' warnings. Fixing this issue might introduce regressions in the package, so the warning is just suppressed.
• debian/patches/03-java21-this-escape.patch: workaround 'this-escape' warning.
• debian/patches/04-java21-explicit-cast.patch: add explicit cast to fix 'lossy-conversion' warning.

Testing:

• rebuild in sid chroot
• rebuild in Java 21 default sid chroot