Fixes#12690
The `strong_migrations` update from f673e3d120
introduced a check for `change_column_null` specific to Postgres. This rejects
old migrations.
This commit just wraps old migrations with `safety_assured` to bypass this
check. Alternatives would have been to:
- Disable that check entirely (a possibility added in that same
`strong_migrations` version) for Mastodon, but it makes sense to write new
migrations without such a strong lock.
- Rewrite the old migrations to do it in a way that do not require an exclusive
lock. I thought fixing those old migrations for performance wasn't worth the
pain. Also, if I understand correctly, the next version of
`strong_migrations` is going to include a helper to do that. We could update
those migrations at that point.
