Fix building on Noble, Debian testing, modernize packaging
- tests: Use en_US.UTF-8 as gettext will ignore LANGUAGE in newer gettext versions
- debian/: update debhelper-compat to 13
- debian/: adapt packaging to be buildable on Debian testing
- debian/: adapt some packaging from Debian where appropriate
- debian/: adapt some packaging from Debian where appropriate
- debian/control: add Breaks:, Replaces:, Provides: on the old pkg name