Re-run background migration for PersonalAccessToken#expires_at

What does this MR do and why?

• V1 was added and run as part of 16.0: !120239 (merged)
• We had logic to ensure that no new tokens were generated with a nil expires_at: !120213 (merged)
• But there was a bypass available: #413289 (closed)
• That bypass was patched via !121370 (merged)
• If we re-run this migration, we should once again and henceforth have zero PATs with a nil expires_at value

Screenshots or screen recordings

How to set up and validate locally

• Create a personal access token with a nil expires at value (needs to be done before migration in !121370 (merged) is run)
• Run this background migration
• All PATs should have expires_at set

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.