Bump gophercloud to v2.13.0 to remove 300GB client-side limit on openstack_db_instance_v1 #2043

Creating openstack_db_instance_v1 with size greater than 300 fails before any request is sent to the OpenStack/Trove API:

Error: Error creating openstack_db_instance_v1: Size (GB) must be between 1-300

This error is not produced by the provider schema or by Terraform/OpenTofu. It comes from client-side validation in gophercloud's instances.CreateOpts.ToInstanceCreateMap().

The fix is already in gophercloud v2.13.0 (PR gophercloud/gophercloud#3769, release). Please bump the dependency from the current v2.12.0 to v2.13.0 so volumes larger than 300GB can be created when the backend allows it.