chore(deps): update dependency google/cloud-pubsub to v2 (#2000)

Author: renovate-bot

functions/helloworld_pubsub/composer.json

@@ -11,7 +11,7 @@
         ]
     },
     "require-dev": {
-        "google/cloud-pubsub": "^1.29",
+        "google/cloud-pubsub": "^2.0",
         "google/cloud-logging": "^1.21"
     }
 }