Update checkout action to v4 to avoid node deprecation (and inform downstream)

[zkamvar]

When I was working on #20, I noticed this deprecation warning:

The following actions uses Node.js version which is deprecated and will be forced to run on node20: actions/checkout@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

We should update these workflows and inform the downstream repositories that they also need to update their workflows.

[annakrystalli]

Sure thing.

Once updated, you could send your first email to the community through our mailing list if you wanted! 😃

Once our workflows have been updated, folks can use hubCI package to re-install the updated versions of workflows.

Also, if we're going to be updating our workflows, what are your thoughts on this comment I added to the #20 PR review?

Follow up question regarding installing our packages in workflows. Until they are on CRAN but given we now have an r-universe, should we be using that to install from instead of GitHub for the time being?

If we want to change the source of our packages, might be a good opportunity to do so.