chore: update release-level in .repo-metadata.json (#145)

Author: Neenu1995

.repo-metadata.json

@@ -4,7 +4,7 @@
  "product_documentation": "https://cloud.google.com/binary-authorization/docs",
  "api_description": " is a service on Google Cloud that provides centralized software supply-chain security for applications that run on Google Kubernetes Engine (GKE) and Anthos clusters on VMware",
  "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-binary-authorization/latest/history",
- "release_level": "beta",
+ "release_level": "preview",
  "transport": "grpc",
  "language": "java",
  "repo": "googleapis/java-binary-authorization",