Finalize BackfillPCiRunnerMachineBuildsProjectId migration

Finalize the BackfillPCiRunnerMachineBuildsProjectId and RetryBackfillPCiRunnerMachineBuildsProjectId migrations introduced in !164386 (diffs) and !168420 (merged).

Documentation: https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#finalize-a-batched-background-migration

Thankfully, the self-managed background migration was added in %17.4, and the .com in %17.5, in time for the %17.5 required stop.