Use API source by default

What does this MR do?

As part of omnibus-gitlab#5993 (closed) we want to change the default to API in %14.0

Users can still explicity set use_legacy_storage=true in omnibus which will set domain-config-source=disk as temporary solution for any problem they face with the new architecture

This commit also leaves disk as default for many acceptance tests because these tests are not yet refactored to use API, see #571 (closed)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines