Merge branch '433172_add_db_constrint_for_member_approval' into 'master'

Adds unique index and model validation on the member_approval table

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144096



Merged-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Approved-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Approved-by: Aakriti Gupta <agupta@gitlab.com>
Reviewed-by: Doug Stull <dstull@gitlab.com>
Reviewed-by: Aakriti Gupta <agupta@gitlab.com>
Reviewed-by: Prabakaran Murugesan <pmurugesan@gitlab.com>
Co-authored-by: Suraj Tripathi <stripathi@gitlab.com>