Remove old migrations in 16.3

In Remove old migrations in 15 (#370640 - closed), we removed all of the pre-15.0 database migrations since that was a required stop. We should do this again to clean out the old migrations.

Deliverables

• Pre-16.3 database migrations are removed
• Remove finalized BBM classed Pre-16.3
• All background migrations from before 16.3 are finalized
• Ensure all squash batch migrations have the pipeline:run-all-e2e label