Nullify merge request metrics user in batches on user deletion

What does this MR do and why?

We have been seeing failures to delete a user because the foreign key relation with merge_request_metrics.merged_by_id and users.id 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 merge_request_metrics.merged_by_id and merge_request_metrics.latest_closed_by_id.

Relates to #358760 (closed)

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