fix: pubsub version constant (#7362)

Author: bshaffer

src/PubSubClient.php

@@ -99,7 +99,7 @@ class PubSubClient
     use ApiHelperTrait;
     use ClientOptionsTrait;
 
-    const VERSION = '2.0.0-RC1';
+    const VERSION = '2.2.0';
 
     const FULL_CONTROL_SCOPE = 'https://www.googleapis.com/auth/pubsub';