Finalize BackfillShardingKeyIdOnCiRunners background migration

BackfillShardingKeyIdOnCiRunners was 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_runners_e59bb2812d partitioned table (!166308 - merged) to backfill the partitioned table.

We should also remove before_validation :ensure_sharding_key_id, on: :update in app/models/ci/runner.rb.