Finalize BackfillRunnerTypeAndShardingKeyIdOnCiRunnerManagers background migration

BackfillRunnerTypeAndShardingKeyIdOnCiRunnerManagers will hopefully be merged in %17.5, which was a required stop. Therefore, we should wait until %17.6 (or the milestone where the migration finishes, if later than %17.6) to finalize the migration. This migration will be required in Create ci_runner_managers partitioned table (#497562 - closed) to backfill the partitioned table.