Use pkgjs/action for CI to auto-upgrade to latest nodes

The CI workflow used here is almost identical to a standard Node.js library testing pattern (checkout, npm install, npm test).

As part of Package Maintenance WG, I've recently created pkgjs/action - it basically does that standard Node testing pattern, but it automatically maintains the test matrix for you, i.e. it will automatically detect the lowest supported (major) version from engines field and will automatically add new Node.js versions as they come out.

As the action is pretty new, I'd like to get some battle testing for it.

Would people mind if I opened a PR to start using it here? I'm willing to take on the maintenance of that until we feel it's stable.

The end result here would probably look something like this: https://github.com/nodejs/undici/blob/main/.github/workflows/nodejs.yml

Start with .github/workflows/nodejs.yml and compare it with the linked undici workflow, then read the pkgjs/action documentation to understand its expected configuration. Done means the citgm workflow uses pkgjs/action and automatically covers the supported and newly released Node.js versions without losing the existing test steps.