Merge branch '413104-allow-merges-from-train-ref' into 'master'

Support Merging the merge train ref directly

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125921



Merged-by: A Browne <abrowne@gitlab.com>
Approved-by: A Browne <abrowne@gitlab.com>
Reviewed-by: Fabio Pitino <fpitino@gitlab.com>
Reviewed-by: Patrick Bajao <ebajao@gitlab.com>
Reviewed-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Reviewed-by: A Browne <abrowne@gitlab.com>
Reviewed-by: Max Fan <mfan@gitlab.com>
Co-authored-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Co-authored-by: Max Fan <mfan@gitlab.com>