fix: use published_at for release sorting

[igorpecovnik]

Summary

• Fix release sorting to use published_at instead of created_at
• Prevents accidental deletion of newer releases when multiple releases have identical creation timestamps

Problem

When releases are created in bulk (e.g., multiple trunk releases created simultaneously), they share the same created_at timestamp. This causes unstable sort order in the delete-old-releases workflow, resulting in newer releases being deleted while older ones are kept.

Solution

Changed sorting from created_at to published_at, which reflects the actual publication order regardless of when the release object was initially created.

Test plan

• Manual verification with dry-run shows correct releases selected for deletion
• Newest releases (by publish date) are preserved
• Older incomplete prereleases are properly cleaned up