Streamline templates (#23107)

Summary:
Clarify the purpose of each issue template. Stop accepting discussions here, as there's a separate repository for that purpose.

Changelog:
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[General] [Changed] - Streamline issue templates on GitHub.
Pull Request resolved: https://github.com/facebook/react-native/pull/23107

Differential Revision: D13777281

Pulled By: hramos

fbshipit-source-id: a5b3e790a974a6898d611f0b60f8626e315e7585