build failed due to deprecated Node.js 16

I got this error message from git action:
```
Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3.
```
I have verified that it can be fixed by upgrading to `actions/checkout@v4` in the .github/workflows/workflow.yaml file.

More an issue with coursera or labs.cognitiveclass.ai, but not sure how to report on them.