Update actions/checkout + actions/setup-node in CI to v3

[striezel]

Updates the actions/checkout and actions/setup-node actions used in the GitHub Actions workflow to their newest major version.

Still using v2 of actions/checkout or actions/setup-node will generate some warning like in this run: https://github.com/typicode/json-server/actions/runs/3689280970

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout@v2, actions/setup-node@v2

The PR will get rid of those warnings for actions/checkout and actions/setup-node, because v3 of both actions uses Node.js 16.