Add Ruby 2.7 images for GitLab CE/EE images

Ruby 2.7.1 is only available on Debian Buster, so we have to upgrade from Debian Stretch.

Fix install-essentials for Debian 10 (Buster):

1. MySQL client is no longer needed
2. Yank libgdbm3 and libgdbm-dev because it's not clear why it's needed
3. Pull legacy nodejs from Debian Buster builds since Node 12.x conflicted