Adds a gitlab:db:migration_testing:down rake task to instrument down
migrations on the current branch.

Renames the gitlab:db:migration_testing rake task to
gitlab:db:migration_testing:up for symmetry

Keeps the gitlab:db:migration_testing task for now, so that this change
is backwards compatible with the migration testing pipeline, which does
not yet know about these changes.

Relates to
https://gitlab.com/gitlab-org/database-team/gitlab-com-database-testing/-/issues/18