Extend ApplicationSettings to support `default_branch_name`
Extend API response to include it, and add tests to show that we can change it.
Look at the API endpoints mentioned in this thread https://gitlab.com/gitlab-org/gitlab/-/issues/221013#note_367733533
Expose the field behind a feature flag
Related to https://gitlab.com/gitlab-org/gitlab/-/issues/221013
Edited by Kerri Miller