Fix and modernize GitHub Actions workflows

[Copilot]

All four workflows had broken or outdated action versions; the Java CI was entirely mismatched to a JavaScript/Node.js project.

Changes

• java-setup.yml → replaced Java CI (referenced nonexistent checkout@v6 / setup-java@v5) with Node.js CI: setup-node@v4 with yarn cache, yarn install --frozen-lockfile, yarn build
• datadog-synthetics.yml → added guard step that emits a warning and skips cleanly when DD_API_KEY/DD_APP_KEY secrets are absent instead of hard-failing
• sign_checker.yaml → checkout@v2 → v4, tj-actions/changed-files@v18.4 → v45
• upload_assets.yaml → checkout@v3 → v4, google-github-actions/auth@v1 → v2, upload-cloud-storage@v1 → v2

Checklist

• Use the owner account that submitted registrar_reserve to sign the project information, and fill in the signature.
• After each update of the project information, please sign the latest data and submit the signature.
• Add a description for your pr, the description includes the original data.
• Fill up the signature of the original data, signature account to Signature Auto check.

Signature Auto check

Para ID Owner & Signature Account:
``

Data that needs to be signed:
file

Signature Hash: ``