Delayed user deletion due to backlog

The rollout of Remove expired and revoke token information aft... (#471683 - closed) removed a considerable number of obsolete bots. See internal numbers. The deleted users are immediately blocked, and many of the associated records are removed immediately, however the rest are cleaned up via a batch job that removes 1000 at a time.

Due to the backlog from obsolete bots, the current batch job has a delay of 2 days to clean up those records such that removals and re-addition of users with identical user names will not see the deletion first complete until the backlog is cleared.

The deletion tasks account for DB resources, and as such should not be scaled up directly. See https://gitlab.com/gitlab-com/support/support-team-meta/-/issues/6368+ for list of users we should start to investigate.