[Documentation] JavaScript Environment Requirements

This issue was originally opened by @EWhite613 in https://github.com/emberjs/ember.js/issues/20471 and transferred to this repository. Copy-pasted content:

With the new [Ember browser support policy](https://emberjs.com/browser-support/) it's difficult to see a way forward with Ember for apps that need to support legacy browser versions outside that statement.

Ember should have a JavaScript Environment Requirements like [React](https://legacy.reactjs.org/docs/javascript-environment-requirements.html) did, and other libraries have. And include in changelogs when new features are used ([react example](https://react.dev/blog/2022/03/08/react-18-upgrade-guide#other-breaking-changes)).

This way those that need to support older legacy browser versions at least have an idea what they need to polyfill (or if they can even polyfill).