fix: removed abandoned package from composer (#1780)

Author: vishwarajanand

pubsub/api/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
         "google/cloud-pubsub": "^1.39",
-        "wikimedia/avro": "^1.9"
+        "rg/avro-php": "^2.0.1||^3.0.0"
     }
 }