Finalize routes backfilling for projects

Ensure project namespace backfilling on routes for projects did finish successfully.

This ensures the original backfilling migration on routes !81121 (merged) has finihsed.

re #358956 (closed)

NOTE:

• This is in draft for now as we are awaiting for the retry migration change request to be run first, see gitlab-com/gl-infra/production#7076 (closed)
• gitlab-com/gl-infra/production#7076 (closed) - is done now.