chore: update release-level in .repo-metadata.json (#976)

Author: Neenu1995

java-pubsub/.repo-metadata.json

@@ -6,7 +6,7 @@
   "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-pubsub/latest/history",
   "api_description": "is designed to provide reliable, many-to-many, asynchronous messaging between applications. Publisher applications can send messages to a topic and other applications can subscribe to that topic to receive the messages. By decoupling senders and receivers, Google Cloud Pub/Sub allows developers to communicate between independently written applications.",
   "issue_tracker": "https://issuetracker.google.com/savedsearches/559741",
-  "release_level": "ga",
+  "release_level": "stable",
   "language": "java",
   "repo": "googleapis/java-pubsub",
   "repo_short": "java-pubsub",