Gradle 6.9, Android Gradle Plugin 4.2.1 (#31593)

Summary:
Bump Gradle to 6.9 which supports Apple Silicon, also Android Gradle Plugin 4.2.1 which defaults to Java 1.8 so no additional config required.

## Changelog

[Android] [Changed] - Bump Gradle to 6.9, Android Gradle Plugin to 4.2.1

Pull Request resolved: https://github.com/facebook/react-native/pull/31593

Test Plan: rn-tester builds and runs as expected

Reviewed By: mdvacca

Differential Revision: D28711942

Pulled By: ShikaSD

fbshipit-source-id: 2a4616cd0f17db7616ab29dea1652717f2cd0f6d