Immediately hard-fail if database names are unknown

The application should hard fail if the config/database.yml contains any unknown database names. In other words, it needs to be a known name:

• main - Already introduced by omnibus-gitlab!5510 (merged) and gitlab-org/charts/gitlab!2105 (merged)
• ci - Will be possible as part of &6271 (closed)
• geo - Will be possible as part of &6458 (closed)
• primary or nil. If the config/database.yml uses the 2 tier config, then it has a nil name, so that's OK, and should be allowed to proceed