Merge branch 'sh-refactor-ff-merge-service' into 'master'

Improve fast-forward merge service reliability

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/97871



Merged-by: charlie ablett <cablett@gitlab.com>
Approved-by: Serhii Yarynovskyi <syarynovskyi@gitlab.com>
Approved-by: charlie ablett <cablett@gitlab.com>
Co-authored-by: Stan Hu <stanhu@gmail.com>