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

packages/react-native-gradle-plugin/build.gradle.kts

@@ -46,8 +46,8 @@ dependencies {
 }
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_11
-    targetCompatibility = JavaVersion.VERSION_11
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
 }
 
 tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {