Fix free shared runners minutes backfill

What does this MR do and why?

Fix free shared runners minutes backfill

Previously the backfill targetted only Namespaces with a type of Group, but needed to also include Users. This changes the backfill to target Users now that Groups have been processed, and re-enqueues the backfill

Previous migration has been completed

References

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• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/443629
• Previous backfill: !161485 (merged)

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