fix: Lower the Subscriber protocol version to disable the streaming pull keepalive feature

Author: michaelpri10

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Subscriber.java

@@ -144,7 +144,7 @@ public class Subscriber extends AbstractApiService implements SubscriberInterfac
   private final boolean maxDurationPerAckExtensionDefaultUsed;
   private final java.time.Duration minDurationPerAckExtension;
   private final boolean minDurationPerAckExtensionDefaultUsed;
-  private final long protocolVersion = 1L;
+  private final long protocolVersion = 0L;
 
   // The ExecutorProvider used to generate executors for processing messages.
   private final ExecutorProvider executorProvider;