chore: update release_level in repo-metadata.json (#165)

* chore: update .repo-metadata.json

* remove api_shortname

Author: parthea

.repo-metadata.json

@@ -2,7 +2,7 @@
     "name": "google-cloud-core",
     "name_pretty": "Google API client core library",
     "client_documentation": "https://cloud.google.com/python/docs/reference/google-cloud-core/latest",
-    "release_level": "ga",
+    "release_level": "stable",
     "language": "python",
     "library_type": "CORE",
     "repo": "googleapis/python-cloud-core",