Cover the full-upgrade scenario in CI (Buster -> Bullseye)
User may hit a case where after a full-upgrade, the MariaDB Server is not working anymore (removed, not started, etc.). See: #993527. This CI step makes sure that after a full-upgrade, the MariaDB Server is still working and running.
This is done by adding a step that will full-upgrade from Buster to Bullseye (installing MariaDB packages from artifacts in the full-upgrade process).