[stable/2023.1 only] Fix incorrect cherry-pick for ext gw

A cherry pick [0] to fix log warnings accidentally had
code that only supported multiple external gateways,
which wasn't merged until 2023.2. Change to conform to
2023.1 code where there is only one external_gateway_info
object in a router.

[0] https://review.opendev.org/c/openstack/neutron/+/908823/

Closes-bug: #2057942
Change-Id: I340bc7a83a65d20ec66edcb505784dfad2372d9f

Author: brianphaley

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/maintenance.py

@@ -978,8 +978,10 @@ def add_gw_port_info_to_logical_router_port(self):
         cmds = []
         context = n_context.get_admin_context()
         for router in self._ovn_client._l3_plugin.get_routers(context):
-            ext_gw_networks = [
-                ext_gw['network_id'] for ext_gw in router['external_gateways']]
+            ext_gw_net_id = (router.get('external_gateway_info') or
+                             {}).get('network_id')
+            if not ext_gw_net_id:
+                continue
             rtr_name = 'neutron-{}'.format(router['id'])
             ovn_lr = self._nb_idl.get_lrouter(rtr_name)
             for lrp in ovn_lr.ports:
@@ -992,7 +994,7 @@ def add_gw_port_info_to_logical_router_port(self):
                 network_id = ovn_network_name.replace('neutron-', '')
                 if not network_id:
                     continue
-                is_ext_gw = str(network_id in ext_gw_networks)
+                is_ext_gw = str(network_id == ext_gw_net_id)
                 external_ids = lrp.external_ids
                 external_ids[ovn_const.OVN_ROUTER_IS_EXT_GW] = is_ext_gw
                 cmds.append(

neutron/tests/unit/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_maintenance.py

@@ -943,7 +943,7 @@ def test_add_gw_port_info_to_logical_router_port(self):
         internal_net_id = uuidutils.generate_uuid()
         routers_db = [{
             'id': uuidutils.generate_uuid(),
-            'external_gateways': [{'network_id': ext_net_id}]}]
+            'external_gateway_info': {'network_id': ext_net_id}}]
         ext_gw_lrp = fakes.FakeOvsdbRow.create_one_ovsdb_row(
             attrs={'external_ids': {
                 constants.OVN_NETWORK_NAME_EXT_ID_KEY: