Make PR template consistent with Changelog (#22117)

Summary:
In the new changelog format, we're following [Keep a Changelog](https://keepachangelog.com/en/1.0.0). This change updates the PR template to follow their change types, plus it simplifies the *CATEGORY* field to follow the changelog's groupings. Some simplified examples have been provided, though I plan to revisit this after 0.58 to add real-world examples. This change is motivated by react-native-community/react-native-releases#47.
Pull Request resolved: https://github.com/facebook/react-native/pull/22117

Differential Revision: D12919271

Pulled By: TheSavior

fbshipit-source-id: 91f063f769357b24383db281545678b924ad3efb