Fix compilation warnings introduced by Gradle 8.4 (#39959)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39959

We're still accessing project.buildDir which will be removed in Gradle 9.0
I'm cleaning it up here.

Changelog:
[Internal] [Changed] - Fix compilation warnings introduced by Gradle 8.4

Reviewed By: yungsters

Differential Revision: D50016573

fbshipit-source-id: de7a725f61b503f08991ebf85b9a002cefab221a

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

@@ -66,6 +66,7 @@ tasks.withType<KotlinCompile>().configureEach {
     apiVersion = "1.5"
     // See comment above on JDK 11 support
     jvmTarget = "11"
+    allWarningsAsErrors = true
   }
 }
 

packages/react-native-gradle-plugin/src/main/kotlin/com/facebook/react/TaskConfiguration.kt

@@ -23,6 +23,7 @@ internal fun Project.configureReactTasks(variant: Variant, config: ReactExtensio
   val targetName = variant.name.capitalizeCompat()
   val targetPath = variant.name
 
+  val buildDir = this.layout.buildDirectory.get().asFile
   // Resources: generated/assets/react/<variant>/index.android.bundle
   val resourcesDir = File(buildDir, "generated/res/react/$targetPath")
   // Bundle: generated/assets/react/<variant>/index.android.bundle