Promote prod to v1.3.9 (modelcontextprotocol#775)

Deploys v1.3.9 to production.

## Changes in v1.3.9
- New web UI for browsing the registry (modelcontextprotocol#757)
- Fixed namespace match validation for websiteUrl and remote URL (modelcontextprotocol#769)
- Added releases administration guide (modelcontextprotocol#764)
- Updated community projects documentation
- Multiple dependency updates

Release notes:
https://github.com/modelcontextprotocol/registry/releases/tag/v1.3.9

The [deploy-production
workflow](https://github.com/modelcontextprotocol/registry/actions/workflows/deploy-production.yml)
will automatically deploy once this PR is merged.

Author: domdomegg

deploy/Pulumi.gcpProd.yaml

@@ -1,7 +1,7 @@
 config:
   mcp-registry:environment: prod
   mcp-registry:provider: gcp
-  mcp-registry:imageTag: 1.3.8  # Set specific image tag for production (change this to deploy different versions)
+  mcp-registry:imageTag: 1.3.9  # Set specific image tag for production (change this to deploy different versions)
   gcp:project: mcp-registry-prod
   mcp-registry:githubClientId: Iv23liUydBbI7Z2Q9bOZ
   mcp-registry:githubClientSecret: