Update test workflow to Node.js 20.x and GitHub Actions v4

[Copilot]

The test workflow was using deprecated Node.js 16.x (EOL Sept 2023) and outdated GitHub Actions v2.

Changes

• Updated Node.js from 16.x to 20.x (current LTS)
• Updated actions/checkout from v2 to v4
• Updated actions/setup-node from v2 to v4

steps:
  - uses: actions/checkout@v4
  - name: Use Node.js ${{ matrix.node-version }}
    uses: actions/setup-node@v4
    with:
      node-version: 20.x
      cache: 'npm'

Original prompt

please fix test workflow, likely need action file to be updated or node version set to latest

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