docs: Bump minimum Node.js version to 16.14 (#54285)
### What?
Docs change.
### Why?
- https://github.com/vercel/next.js/issues/54269
### How?
Bump the Node.js version shown at: https://nextjs.org/docs/app/building-your-application/upgrading/app-router-migration#nodejs-version
Fixes #54269
