Deb: Update Salsa-CI to use Bullseye instead of Sid

Tests that try to upgrade MariaDB 10.6 in Debian Sid can no longer work
properly on versions < 10.11 as MariaDB 10.11 in now Debian Sid.

Change RELEASE to use Bullseye and refactor builds and upgrade tests to
use primarily Bullseye, as it has MariaDB 10.5 and thus testing upgrades
to MariaDB 10.6 and higher can work. Add on new 'build sid' job to continue
at least on build on Sid as well, even though it is not tested in other
jobs.

Due to this many Sid specific workarounds are can also be dropped, and
since Bullseye is now used for everything, the old bullseye-backports jobs
are obsolete and removed.

Tests that upgrade MySQL in Sid to MariaDB are also removed, as no test
in Debian Sid with MariaDB < 10.11 can reliably work anymore.

Also disable reprotest as unnecessary on old branches, refactor the naming
of autobake-deb.sh and native Debian build jobs to be more clear.

NOTE TO MERGERS: This commit is made on 10.6 branch and can be merged to
all later branches (10.7, 10.8, ..., 11.0). If/when some jobs break, they
will be fixed per branch on follow-up commits.