Add changelog entry with @babel/polyfill upgrade instructions

[jakeNiemiec]

Taking @connorshea's suggestion from #2031 (comment) to warn users who were using @babel/polyfill. With #2031 merged, users who upgrade will need to remove import '@babel/polyfill' from their pack files and add the replacements to those files and their package.json.

[jaredbeck]

In each of your /packs/*.js files, change this:
import "@babel/polyfill";

What if we don't have such an import? Should we still add the new imports?

[PikachuEXE]

If you use polyfill differently you can skip it
My project use feature detection and only load polyfill chunk if any feature missing

[jakeNiemiec]

I would add the new imports, the docs say that the new imports can handle both core-js 2 & 3 https://babeljs.io/blog/2019/03/19/7.4.0#migration-from-core-js-2.

It's hard to make a general recommendation since all of this depends on how you use babel.config.js. I expect issues to pop up, babel has always been that tool that you forget about until it stops working.