PSL 6.0.3

No code changes. This release improves the release infrastructure.

What changed

Annotated tags: Split repository tags are now created as annotated tags via the GitHub API, removing the "unverified" warning shown on 6.0.0-6.0.2 tags.
Immutable tags: All 62 repositories now have tag immutability rulesets. Tags cannot be deleted, updated, or force-pushed.
Maintenance branch sync: The splitter now syncs the maintenance branch (e.g. 6.0.x) to the tag before splitting, ensuring split repos always receive the correct commits for patch releases.

See CHANGELOG.md for details.