Upgrade Action to use node 16 instead of node 12

[ericcornelissen]

Following the recent blog post "GitHub Actions: All Actions will begin running on Node16 instead of Node12" by GitHub, I noticed this action is still running on node12:

npm-publish/action.yml

Lines 58 to 60 in e42e372

	runs:
	using: node12
	main: dist/index.js

It should be fairly easy to change this by replacing using: node12 with using: node16 - I wouldn't expect anything in this Action's source code to break as a result of that.

One thing to note, by and large I've seen other Actions use a major version bump when releasing this change. The motivation being that older GitHub Actions runners may not support node16. See for example the "What's new" section of actions/checkout.

[razor-x]

I've opened a PR with this update: #68.