deploy: update prod to v1.7.7

[rdimitrov]

Summary

• Bump mcp-registry:imageTag from 1.7.6 → 1.7.7 in deploy/Pulumi.gcpProd.yaml
• v1.7.7 is a security patch covering three findings (validator hardening, attribute-context UI escaping, IPv6 SSRF blocklist extension); see the v1.7.7 release notes
• Staging is on f5f40bd (the v1.7.7 commit) and verified end-to-end:
  • /v0.1/version reports the new commit
  • /v0.1/validate rejects a poisoned websiteUrl with website-url-invalid-characters
  • / HTML serves the escapeAttr helper

Test plan

• Confirm release workflow's ko-push job has finished publishing ghcr.io/modelcontextprotocol/registry:1.7.7 before merging
• Watch deploy-production workflow rolls the registry pods to 1.7.7
• Hit https://registry.modelcontextprotocol.io/v0.1/version and confirm git_commit matches f5f40bd...
• Spot-check the catalogue UI loads cleanly

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