Merge branch 'jnutt/put-endpoint-behind-flag' into 'master'

Place relation import endpoint behind feature flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149444



Merged-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Approved-by: Sam Word <sword@gitlab.com>
Approved-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Co-authored-by: James Nutt <jnutt@gitlab.com>