RFC repo-updater: Do not delete repos in EnabledStateDeprecationMigration

Created by: keegancsmith

We would only run the delete step in the migration. This meant that in the case of us changing the external_id we would delete repositories and only re-add them once the syncer runs. This lead to a customer having all repositories deleted => our gitserver janitor job deleting all clones. This caused downtime due to the slow time it took to reclone everything.

Alternatively we could do a full upsert from the diff.