packages/react-native-gradle-plugin/ -> packages/gradle-plugin/ (#44832)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44832

I'm renaming this folder as now we have 2 gradle plugins + we currently have
`package/react-native-gradle-plugin/react-native-gradle-plugin/` which is confusing so we can just call this folder `packages/gradle-plugin/`
to be consistent with the NPM package name

Changelog:
[Internal] [Changed] - packages/react-native-gradle-plugin/ -> packages/gradle-plugin/

Reviewed By: blakef

Differential Revision: D58284883

fbshipit-source-id: 5a7bb40a5d80f6fbab4ffb29e44107453f1013ec