alembic: Fix compatibility with SQLAlchemy 2.0+.

alembic: Fix compatibility with SQLAlchemy 2.0+.

SQLAlchemy 2.0 changed the way that commits/rollbacks are handled
causing the final `UPDATE` to our `alembic_version_<whatever>` tables
to be rolled back instead of committed.

We now use one connection to determine which
`alembic_version_<whatever>` table to use and another to run the
actual migrations. This prevents the erroneous rollback.

This change is compatible with both SQLAlchemy 1.4 and 2.0.