Fixes #12690

The `strong_migrations` update from ba2eac8824
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.