Avoid imposing alembic 6.0 requirement on all distros

Related-to: LP 1224666

Two of the alembic-based migrations use the new 'type_' kwarg in calls
to op.drop_constraint(), even though the old 'type' form was retained
for backward compatability:

  https://alembic.readthedocs.org/en/latest/ops.html

  "Changed in version 0.5.0: The type parameter is now named type_.
  The old name type will remain for backwards compatibility."

To avoid needlessly imposing the alembic 6.0 on distros for which this
version is not yet packaged, we revert these migrations to use the older
form.

The corresponding change to the global requirements is proposed here:

  https://review.openstack.org/48227

Change-Id: I3796ffa3de0e0f558f368d7ebf8e55c87657753d
Merge "Avoid imposing alembic 6.0 requirement on all distros"