Batch delete/nullify all the missing users associations

We identified failures in user deletion through the issue in part because the foreign key relation with following user associations is failing to nullify within the 15-second statement timeout. Just as we did with nullifying IDs in batches with the issues table in !84709 (merged), we need do the same with.

  1. push_event_payloads.event_id
  2. merge_request_assignees.user_id
  3. resource_state_events.user_id
  4. todos.user_id
  5. todos.author_id
  6. events.author_id.

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