fix(pubsub): update the API

#### pubsub:v1

The following keys were changed:
- schemas.Subscription.properties.expirationPolicy.description

discovery/pubsub-v1.json

@@ -1424,7 +1424,7 @@
       }
     }
   },
-  "revision": "20210816",
+  "revision": "20210824",
   "rootUrl": "https://pubsub.googleapis.com/",
   "schemas": {
     "AcknowledgeRequest": {
@@ -2021,7 +2021,7 @@
         },
         "expirationPolicy": {
           "$ref": "ExpirationPolicy",
-          "description": "A policy that specifies the conditions for this subscription's expiration. A subscription is considered active as long as any connected subscriber is successfully consuming messages from the subscription or is issuing operations on the subscription. If `expiration_policy` is not set, a *default policy* with `ttl` of 31 days will be used. The minimum allowed value for `expiration_policy.ttl` is 1 day."
+          "description": "A policy that specifies the conditions for this subscription's expiration. A subscription is considered active as long as any connected subscriber is successfully consuming messages from the subscription or is issuing operations on the subscription. If `expiration_policy` is not set, a *default policy* with `ttl` of 31 days will be used. The minimum allowed value for `expiration_policy.ttl` is 1 day. If `expiration_policy` is set, but `expiration_policy.ttl` is not set, the subscription never expires."
         },
         "filter": {
           "description": "An expression written in the Pub/Sub [filter language](https://cloud.google.com/pubsub/docs/filtering). If non-empty, then only `PubsubMessage`s whose `attributes` field matches the filter are delivered on this subscription. If empty, then no messages are filtered out.",

src/apis/pubsub/v1.ts

@@ -564,7 +564,7 @@ export namespace pubsub_v1 {
      */
     enableMessageOrdering?: boolean | null;
     /**
-     * A policy that specifies the conditions for this subscription's expiration. A subscription is considered active as long as any connected subscriber is successfully consuming messages from the subscription or is issuing operations on the subscription. If `expiration_policy` is not set, a *default policy* with `ttl` of 31 days will be used. The minimum allowed value for `expiration_policy.ttl` is 1 day.
+     * A policy that specifies the conditions for this subscription's expiration. A subscription is considered active as long as any connected subscriber is successfully consuming messages from the subscription or is issuing operations on the subscription. If `expiration_policy` is not set, a *default policy* with `ttl` of 31 days will be used. The minimum allowed value for `expiration_policy.ttl` is 1 day. If `expiration_policy` is set, but `expiration_policy.ttl` is not set, the subscription never expires.
      */
     expirationPolicy?: Schema$ExpirationPolicy;
     /**