Set Java source/target compatibility for react-native-gradle-plugin to 8

Summary:
This is necessary otherwise when building from source on JVM < 11, the `compileJava`
task of the Gradle Plugin will fail with `invalid source: 11`.
Essentially the Gradle build will not even start because of this. Instead we delegate
to a better formatted warning from either AGP or from our plugin.

Changelog:
[Internal] [Changed] - Set Java source/target compatibility for react-native-gradle-plugin to 8

Reviewed By: ShikaSD

Differential Revision: D34111799

fbshipit-source-id: 57ab11fe6c4532576776b586f75e8fcb5c71adcd