Migrate GitHub Actions off Node 20 runtimes

[ClaudiaFang]

Goal

Remove remaining GitHub Actions that still execute on the deprecated Node.js 20 runtime across active firstsun-dev repositories.

Confirmed affected workflows

• git-files-sync/.github/workflows/ci.yml: dorny/paths-filter v3 -> v4
• heaven-monorepo/.github/workflows/backup-d1.yml: actions/setup-node@v4 -> v6
• .github/.github/workflows/rclone-sync.yml: actions/checkout@v4 -> v6
• skills/.github/workflows/upload-to-gdrive.yml: actions/checkout@v4 -> v6
• books-mgmt/.github/workflows/sync.yml: actions/checkout@v4 -> v6
• books-mgmt/.github/workflows/gdrive_sync.yml: actions/checkout@v4 -> v6
• news-getter/.github/workflows/data-fetch.yml: actions/checkout@v4 -> v6
• news-getter/.github/workflows/build-deploy.yml: actions/checkout@v4, actions/setup-node@v4 -> v6
• windmill-flows/.github/workflows/deploy-windmill.yml: actions/checkout@v4 -> v6
• infra-config/.github/workflows/build-windmill-worker.yml: Docker actions on Node 20 generations -> current Node 24 generations
• heaven-video-summary/.github/workflows/pipeline.yml: astral-sh/setup-uv@v5 -> v8

Constraints

• Do not change application/runtime Node versions unless required for action compatibility.
• Do not change workflow behavior, job DAGs, secrets, permissions, runner selection, deployment logic, or release behavior.
• Keep existing SHA-pinning conventions where already used.
• Validate each changed workflow with actionlint or equivalent YAML/workflow validation and run the repository's relevant CI.

Tracking

Use branch chore/github-actions-node24-migration in each affected repository and reference this issue from every PR.