Update GitHub Actions before Node 20 runner deprecation

[andreasronge]

Context

The v0.12.0 release workflow emitted GitHub Actions warnings that several JavaScript actions still run on Node.js 20 internally.

This is CI plumbing only; it does not affect the Elixir/BEAM runtime or ptc_runner package behavior.

Warning

GitHub says Node.js 20 actions are deprecated. Node 24 becomes the default on 2026-06-16, and Node 20 is removed from runners on 2026-09-16.

Affected Actions Observed

The release run warned for actions such as:

• actions/checkout@v4
• actions/upload-artifact@v4
• actions/download-artifact@v4
• actions/cache@v4

These appear in .github/workflows/release.yml and shared workflow helpers such as .github/actions/setup-elixir/action.yml, with additional usages across other workflows.

Proposed Work

• Audit all workflow/action references under .github/.
• Upgrade GitHub-managed actions to versions that declare Node 24 support when available/current.
• Run representative workflows or at least the release dry run after updating.
• Keep this scoped to CI maintenance; no product/runtime changes expected.

Verification

A successful fix should remove the Node 20 deprecation annotations from release/test workflows.

[andreasronge]

Fixed on main in a8f8f11.\n\nUpdated GitHub-managed actions across .github to Node 24-compatible majors and added workflow maintenance guidance for future runtime deprecations.\n\nVerified with:\n- mix precommit\n- actionlint -shellcheck=\n- git diff --check\n- independent Codex review\n- pre-push tests and dialyzer via the repo hook