Finalize backfill of packages_tags.project_id

What does this MR do and why?

Cleanup for !135451 (merged):

• Finalise the BackfillPackagesTagsProjectId batched background migration, which has completed (internal).
• Validate the previously created NOT NULL constraint.

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.

Related to #429173 (closed)