Update Actions to Node.js 20

[jugglinmike]

This project currently defines two "dispatch" Actions, and they both use Node.js 18. GitHub issues the following warning when they are executed:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: guidepup/setup-action@0.15.3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

It's not clear why GitHub reports that Node.js 16 is in use, but in any event, we should follow the recommendation to update to Node.js 20.

[outofambit]

closed by #29