Revert the migration for finalizing the conversion to bigint for push_event_payloads

We have not enabled the cron job for BatchedBackgroundMigrationWorker on self-managed instances in %14.0, which results to the error mentioned in gitlab-org/charts/gitlab#2785 (closed) when an instance upgrades to a higher (minor) version that includes a column swap migration (e.g. !64577 (merged))

Our plan to resolve this is the following:

1. Enable the cron job for BatchedBackgroundMigrationWorker on self-managed instances and backport the change to %14.0 as well (#334996 (closed))

2. Revert the column swap migration for push_event_payloads (!64577 (merged)) on .com and do not release the migration to self managed instances.

3. Release all the column swap changes on %14.2 onwards

   That makes %14.1 a safe release, with the batched background migrations able to automatically run and finish the backfilling of all bigint columns.

4. Require that all self-managed instance upgrade through %14.1

   That will allow instances to catchup, finish all the pending migrations and be ready for the column swap once they upgrade to the next version they plan to upgrade

In this issue we'll track the efforts to revert the the column swap migration for push_event_payloads (!64577 (merged)) on Gitlab.com without releasing any migrations for self-managed instances in %14.1

The plan follows option 2 that @krasio proposed in #288005 (comment 614744621)