[P2] Migrate GitHub Actions from Node.js 20 to Node.js 24

[phranck]

Problem

GitHub Actions emits Node.js 20 deprecation annotations in the CI workflow. The warnings currently identify the pinned actions/checkout@v4.3.1, actions/upload-artifact@v4.6.2, and actions/download-artifact@v4.3.0 revisions in .github/workflows/ci.yml.

GitHub-hosted runners currently force these actions onto Node.js 24, so CI remains green, but the pinned revisions are deprecated and may stop working once compatibility enforcement changes.

Proposed solution

• Update every checkout, artifact upload, and artifact download reference to a current stable Node.js 24-backed release.
• Review the major-version migration notes and preserve all existing checkout and artifact behavior.
• Continue pinning actions by full commit SHA and update the adjacent version comments.
• Keep the permissions model and deploy flow unchanged.

Acceptance criteria

• No CI job reports a Node.js 20 deprecation annotation.
• macOS, Linux, SwiftUI snapshots, API compatibility assembly, test badge update, and DocC deployment remain functional.
• Actionlint and the repository CI-configuration validation pass.
• All third-party actions remain pinned by full commit SHA.

Parallelization

This can be implemented independently of SDK and API work. It only conflicts with other changes to .github/workflows/ci.yml.