Merge branch 'new-keep-to-remove-old-feature-flag' into 'master'

Add a new Keeps::DeleteOldFeatureFlags keep

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/140437



Merged-by: Peter Leitzen <pleitzen@gitlab.com>
Approved-by: Peter Leitzen <pleitzen@gitlab.com>
Reviewed-by: Rémy Coutable <remy@rymai.me>
Reviewed-by: Peter Leitzen <pleitzen@gitlab.com>
Co-authored-by: Rémy Coutable <remy@rymai.me>
Co-authored-by: Dylan Griffith <dyl.griffith@gmail.com>