Deprecate support for openSUSE Leap 15.2
openSUSE Leap 15.2 has its EOL on December 2021. Which makes GitLab 14.6 the last version we can provide openSUSE Leap 15.2 packages for. After that, we should stop building packages for it.
NOTE: This requires us to do #6426 (closed) first to provide users a migration path.