Bump Community CLI to 21.0.0-alpha.1

[huntie]

Summary

Follows #235, required to unblock react/react-native#58671.

Community CLI 21.x will bundle the iOS and Android platforms — a hard expectation of the template with #235's changes.

Follow-ups:

• Bump to stable CLI 21.x here when released.

Changelog:
[General][Changed] - Bump @react-native-community/cli to 21.0.0-alpha.1

Test Plan

Template-shaped app (this template's android/ and ios/, react-native from react/react-native#58671, no react-native.config.js), running the Gradle plugin's autolinking command:

npx @react-native-community/cli config | jq '{platforms: (.platforms | keys), androidPackage: .project.android.packageName, iosProject: .project.ios.xcodeProject.name}'

CLI	platforms	project.android.packageName	project.ios
20.2.0	none	null	null
21.0.0-alpha.1	android, ios	com.helloworld	HelloWorld.xcworkspace

• ✅ With 21.0.0-alpha.1, the platforms and project config come from the CLI
• ℹ️ With 20.2.0, the config the template-app CI jobs in Remove Community CLI platform config (upstreamed) (2/2) react/react-native#58671 currently fail on: RNGP - Autolinking: Could not find project.android.packageName in react-native config output!