chore: update release-level in .repo-metadata.json (#61)

Author: Neenu1995

.repo-metadata.json

@@ -4,7 +4,7 @@
  "product_documentation": "https://cloud.google.com/deploy/docs",
  "api_description": "is a service that automates delivery of your applications to a series of target environments in a defined sequence",
  "client_documentation": "https://googleapis.dev/java/google-cloud-deploy/latest/index.html",
- "release_level": "beta",
+ "release_level": "preview",
  "transport": "grpc",
  "language": "java",
  "repo": "googleapis/java-deploy",