Fix incompatibility with google-cloud-monitoring 2.18.0 (#36200)

Google cloud monitoring 2.18.0 released yesterday broke our tests.
Upgrading minimum version and fixing the tests shoudl fix the
problem.

(cherry picked from commit 4c73d61)

Author: potiuk

airflow/providers/google/provider.yaml

@@ -111,7 +111,7 @@ dependencies:
   - google-cloud-language>=2.9.0
   - google-cloud-logging>=3.5.0
   - google-cloud-memcache>=1.7.0
-  - google-cloud-monitoring>=2.14.1
+  - google-cloud-monitoring>=2.18.0
   - google-cloud-orchestration-airflow>=1.10.0
   - google-cloud-os-login>=2.9.1
   - google-cloud-pubsub>=2.19.0

generated/provider_dependencies.json

@@ -440,7 +440,7 @@
       "google-cloud-language>=2.9.0",
       "google-cloud-logging>=3.5.0",
       "google-cloud-memcache>=1.7.0",
-      "google-cloud-monitoring>=2.14.1",
+      "google-cloud-monitoring>=2.18.0",
       "google-cloud-orchestration-airflow>=1.10.0",
       "google-cloud-os-login>=2.9.1",
       "google-cloud-pubsub>=2.19.0",

tests/providers/google/cloud/operators/test_stackdriver.py

@@ -112,6 +112,7 @@ def test_execute(self, mock_hook):
                 "display_name": "",
                 "name": "test-name",
                 "notification_channels": [],
+                "severity": 0,
                 "user_labels": {},
             }
         ] == result