Remove Community CLI platform config (upstreamed) (2/2)

[huntie]

Summary:

Replaces #57353.

Depends on #58670 and react-native-community/cli#2817. See also react-native-community/template#235.

Note

This PR includes the commit from #58670 until that lands. Review the top commit only.

Deletes the react-native.config.js file we ship with react-native. The iOS and Android Community CLI platforms are moving back to being included by default (and in the first place, should not have been wired up here!), and all commands are self-registered by @react-native/community-cli-plugin.

RNTester, pinned to @react-native-community/cli 20.2.0, registers the iOS and Android platforms in its own react-native.config.js until it moves to 21.x.

Changelog:
[General][Changed] - Platform registration config for the Community CLI is removed from the react-native package. No breaking change with @react-native-community/cli 21.x (required).

Test Plan:

RNTester

• ✅ react-native config commands, platforms, and project config are unchanged from main, with both CLI 20.2.0 and the breaking: Bundle iOS and Android as core platforms react-native-community/cli#2817 build
• ✅ react-native bundle --platform ios --dev false completes with the Fix tiny grammatical error #2817 build

Template-shaped app (no react-native.config.js in the project or in react-native)

• ✅ With the Fix tiny grammatical error #2817 build: android and ios platforms from the CLI, plus the plugin's bundle, start, spm, and codegen
• ✅ With CLI 20.2.0: no platforms, confirming 21.x is required

Screenshots of the template app are in the react-native-community/cli#2817 test plan.