docs: update documentation for 31 day subscription message retention

PiperOrigin-RevId: 673028761

google/pubsub/v1/pubsub.proto

@@ -837,7 +837,7 @@ message Subscription {
   // backlog, from the moment a message is published. If `retain_acked_messages`
   // is true, then this also configures the retention of acknowledged messages,
   // and thus configures how far back in time a `Seek` can be done. Defaults to
-  // 7 days. Cannot be more than 7 days or less than 10 minutes.
+  // 7 days. Cannot be more than 31 days or less than 10 minutes.
   google.protobuf.Duration message_retention_duration = 8
       [(google.api.field_behavior) = OPTIONAL];