feat: update to node.js 20

I get the following warning in the Actions tab:

```
Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: exuanbo/actions-deploy-gist@v1. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
```