Fix Automated Package Cleanup cut-off on main

[haoruizhou]

Summary

• Scheduled Automated Package Cleanup on main has been failing every Sunday because cut-off: 5 versions is not a valid snok/container-retention-policy@v2 input (dateparser requires a timezone-aware datetime like 4 weeks ago UTC).
• Keep the existing keep-at-least: 5 behavior; only correct cut-off.

Test plan

• After merge, run Automated Package Cleanup via workflow_dispatch and confirm jobs get past input validation
• Confirm deletions still respect keep-at-least: 5 and skip-tags: latest, test-latest