ci: migrate release/CI workflows off Node.js 20 actions

[hidetzu]

Motivation

GitHub Actions runners will switch to Node.js 24 by default on 2026-06-02, and Node.js 20 will be removed on 2026-09-16. The current workflows depend on actions still pinned to Node.js 20:

• `actions/checkout@v4`
• `mlugg/setup-zig@v2`
• `softprops/action-gh-release@v2`

Today these emit deprecation warnings on every run (visible in the v0.2.0 release run); they will silently fail (or be forced into a compatibility mode) once the cutover happens.

Tasks

• Check whether each action has a newer release that runs on Node.js 24, and upgrade if available.
• If no upgrade exists yet, opt in early via `FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true` on the affected workflow(s) so we surface failures before the forced cutover.
• Verify the CI workflow (`.github/workflows/ci.yml`) and release workflow (`.github/workflows/release.yml`) both pass after the change.
• Confirm a tag-driven release run still produces all 4 platform assets (Linux glibc, macOS Intel, macOS Apple Silicon, Windows).

References

• Deprecation notice: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/
• Sample warning observed in run: https://github.com/hidetzu/zemo/actions/runs/25411371086

Not urgent — purely preventative work before the 2026-06-02 cutover.