chore: update release_level in repo-metadata.json (#51)

* chore: update .repo-metadata.json

* revert

* remove api_shortname

Author: parthea

.repo-metadata.json

@@ -2,7 +2,7 @@
     "name": "db-dtypes",
     "name_pretty": "Pandas Data Types for SQL systems (BigQuery, Spanner)",
     "client_documentation": "https://googleapis.dev/python/db-dtypes/latest/index.html",
-    "release_level": "beta",
+    "release_level": "preview",
     "language": "python",
     "library_type": "INTEGRATION",
     "repo": "googleapis/python-db-dtypes-pandas",