Update publish workflow to use Node 20

[Copilot]

Bug Fix

The publish workflow was still pinned to Node 14, while the publish command now depends on a newer npm that supports provenance-aware publishing. This change updates only the workflow runtime called out in the review comment.

• What was the bug?

  The npm publish job used an outdated Node/npm toolchain, which could cause the release workflow to fail when running the publish step.

• How did you fix it?

  The publish workflow now uses Node 20 for the publish-npm job, aligning it with the repository’s current CI runtime and ensuring the bundled npm supports the publish command used there.

• Change

  - uses: actions/setup-node@v3
    with:
      node-version: 20
      registry-url: https://registry.npmjs.org/
      cache: npm