Merge branch '424097-ff-cleanup-write_project_authorizations_is_unique' into 'master'

Clean up `write_project_authorizations_is_unique` feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/131866



Merged-by: Roy Zwambag <rzwambag@gitlab.com>
Approved-by: Eulyeon Ko <5961404-euko@users.noreply.gitlab.com>
Approved-by: Roy Zwambag <rzwambag@gitlab.com>
Approved-by: Imre Farkas <ifarkas@gitlab.com>
Co-authored-by: Dominic Bauer <dbauer@gitlab.com>