Introduce group-level protected branches

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167753



Merged-by: Douglas Barbosa Alexandre <dbalexandre@gmail.com>
Approved-by: david hamp-gonsalves <dhamp-gonsalves@gitlab.com>
Approved-by: Douglas Barbosa Alexandre <dbalexandre@gmail.com>
Reviewed-by: Joe Woodward <jwoodward@gitlab.com>
Co-authored-by: Joe Woodward <j@joewoodward.me>