chore: update release-level in .repo-metadata.json (#795)

Author: Neenu1995

.repo-metadata.json

@@ -4,7 +4,7 @@
  "api_description": "provides a Google Cloud Storage extension for Java's NIO Filesystem.",
  "product_documentation": "https://cloud.google.com/storage/docs",
  "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-nio/latest/history",
- "release_level": "beta",
+ "release_level": "preview",
  "language": "java",
  "repo": "googleapis/java-storage-nio",
  "repo_short": "java-storage-nio",