CI: move Node.js workflow to Node 20 and Actions v4 to unblock node-gyp configure

[Copilot]

The workflow in PR #30 fixed the C++ standard issue but then failed later at node-gyp configure because CI was still pinned to Node 16 while the test step installs global node-gyp@13 (which requires newer web globals such as ReadableStream).
This PR updates the workflow runtime and action versions so the existing build/test commands run under a compatible Node LTS.

• Workflow runtime alignment

  • Updated both jobs in .github/workflows/main.yml from node-version: 16 to node-version: 20.

• GitHub Actions version refresh

  • Updated both jobs from actions/checkout@v3 to actions/checkout@v4.
  • Updated both jobs from actions/setup-node@v3 to actions/setup-node@v4.

• Command semantics preserved

  • Kept the test job command sequence unchanged (npm i node-gyp -g, npm install, node-gyp configure, node-gyp build, smoke test command).
  • Kept publish job behavior unchanged aside from runtime/action version updates.

- uses: actions/checkout@v4
- uses: actions/setup-node@v4
  with:
    node-version: 20
    registry-url: https://registry.npmjs.org/