oobmigration: Restore previous frontend crashloop behavior

Created by: efritz

https://github.com/sourcegraph/sourcegraph/pull/29059 moves the oobmigrator from the worker to the frontend. This is fine short-term (as nothing is deprecated in the next release so this functionality won't be hit by anyone), but we should either:

• mirror the migration runner construction but only use validation methods in the frontend
• communicate to the frontend via the database to refuse to start when the runner is invalid for the version

The risk of leaving the behavior as it is after #29059 is that site-admins will see a decreasingly ususable frontend with crashing workers refusing to start. By the design of oob migrations, this failure should be much louder.