Make Project's only_mirror_protected_branches value false by default
This default_value_for
entry didn't actually do anything because by
default, the UI initialized the flag to false
and the DB allows nil
values. However, this ensures the model is consistent with that
behavior and avoids the pitfalls we saw in
https://gitlab.com/gitlab-org/gitlab-ce/issues/53515.
This is similar to the change made in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23410 for remote mirrors.