chore: Update discovery artifacts (#1379)

## Deleted keys were detected in the following stable discovery artifacts:
servicemanagement v1 https://github.com/googleapis/google-api-python-client/commit/ac046117b12b43e03e748ebd0c442b340ed99183

## Discovery Artifact Change Summary:
feat(adexchangebuyer2): update the api https://github.com/googleapis/google-api-python-client/commit/e1ebe49bc9e2e007c61188202f0345c96083c19a
feat(displayvideo): update the api https://github.com/googleapis/google-api-python-client/commit/4f37d8b0d43f2172fdae627605947b3ba15b06c2
feat(gkehub): update the api https://github.com/googleapis/google-api-python-client/commit/e12ff0958ed9016356c8711039a15334d2f0d6f6
feat(servicemanagement): update the api https://github.com/googleapis/google-api-python-client/commit/ac046117b12b43e03e748ebd0c442b340ed99183
feat(spanner): update the api https://github.com/googleapis/google-api-python-client/commit/43f1be2ea175a79e463d77cf3155e7b8ab1b26f6
diff --git a/googleapiclient/discovery_cache/documents/gkehub.v1alpha.json b/googleapiclient/discovery_cache/documents/gkehub.v1alpha.json
index b1d653d..690588e 100644
--- a/googleapiclient/discovery_cache/documents/gkehub.v1alpha.json
+++ b/googleapiclient/discovery_cache/documents/gkehub.v1alpha.json
@@ -670,7 +670,7 @@
       }
     }
   },
-  "revision": "20210512",
+  "revision": "20210521",
   "rootUrl": "https://gkehub.googleapis.com/",
   "schemas": {
     "AuditConfig": {
@@ -1069,6 +1069,22 @@
             "Deployment was attempted to be installed, but has errors"
           ],
           "type": "string"
+        },
+        "gatekeeperMutation": {
+          "description": "Status of the pod serving the mutation webhook.",
+          "enum": [
+            "DEPLOYMENT_STATE_UNSPECIFIED",
+            "NOT_INSTALLED",
+            "INSTALLED",
+            "ERROR"
+          ],
+          "enumDescriptions": [
+            "Deployment's state cannot be determined",
+            "Deployment is not installed",
+            "Deployment is installed",
+            "Deployment was attempted to be installed, but has errors"
+          ],
+          "type": "string"
         }
       },
       "type": "object"
@@ -1351,6 +1367,10 @@
           "description": "Logs all denies and dry run failures.",
           "type": "boolean"
         },
+        "mutationEnabled": {
+          "description": "Enable users to try out mutation for PolicyController.",
+          "type": "boolean"
+        },
         "referentialRulesEnabled": {
           "description": "Enables the ability to use Constraint Templates that reference to objects other than the object currently being evaluated.",
           "type": "boolean"