deploy: update prod to v1.7.6 (#1232)

## Summary

- Bumps prod imageTag from \`1.7.5\` to \`1.7.6\`.

## Test plan

- [ ] Smoke staging at https://staging.registry.modelcontextprotocol.io
- [ ] Roll out via Pulumi
- [ ] Confirm prod health post-rollout: \`curl
https://registry.modelcontextprotocol.io/v0/version\` shows \`1.7.6\`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/Pulumi.gcpProd.yaml

@@ -1,7 +1,7 @@
 config:
   mcp-registry:environment: prod
   mcp-registry:provider: gcp
-  mcp-registry:imageTag: 1.7.5  # Set specific image tag for production (change this to deploy different versions)
+  mcp-registry:imageTag: 1.7.6  # Set specific image tag for production (change this to deploy different versions)
   gcp:project: mcp-registry-prod
   mcp-registry:githubClientId: Iv23liUydBbI7Z2Q9bOZ
   mcp-registry:githubClientSecret: