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

[dominykas]

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

[Trott]

@nodejs/citgm

[BethGriggs]

I was going to open a PR to remove Node.js 12 from the CI workflow, but then realised this would be a preferable approach (IMO).

+1 to doing this.

[dominykas]

Opened a #907, although I'm thinking if it is enough of an improvement if the same is not applied to the test-module workflow... and I need to think about whether a feature to check out an arbitrary repo via pkgjs/action makes sense... Maybe?