Keep latest artifact for ref

What does this MR do?

Related #16267 (closed)

Keep the latest artifact for any given ref regardless of its expiry time.

This adds a locked column to the ci_job_artifacts table and changes the expiry mechanism to delete only artifacts that are not locked. The latest artifact for any ref is locked on creation.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• N/A Separation of EE specific content