v3 upgrade Docs needs to be updated

[jimmywarting]

node-fetch/docs/v3-UPGRADE-GUIDE.md

Line 22 in 5f0500d

## Minimum supported Node.js version is now 10.16

I'm not a native english speaker
So I would like it if someone who is better than me in english updates the document to say that we require node v12.20 now (cuz of ESM #1141)

I also think there needs to be a upgrade guide on how to import node-fetch now when it's a ESM only package from a cjs package.
one way to load it from cjs is via the async import() another way if you do not want to convert you entire project into esm would be to just add a wrapper and load cjs files from a esm file (ref: #1127 (comment))

We do not have to push a new minor patch. Since there is no api changes

Maybe should say something more about this in the main README file under https://github.com/node-fetch/node-fetch#loading-and-configuring-the-module

Who wants to tackle this API documentation?