Remove old migrations in 15

In Remove old migrations in release 14 (#331248 - closed), we removed all of the pre-14.0 database migrations since that was a required stop. We should do this again to clean out the old migrations.

For reference, we are squashing migrations up to 20220517144749.

Deliverables

• Pre-15.0 database migrations are removed
• All background migrations from before 15.0 are finalized