Migrate repo to use JVM toolchains Gradle feature.

^KT-46972 Fixed

core/descriptors.runtime/build.gradle.kts

@@ -1,10 +1,24 @@
-import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+import org.jetbrains.kotlin.gradle.tasks.UsesKotlinJavaToolchain
 
 plugins {
     kotlin("jvm")
     id("jps-compatible")
 }
 
+// Only compilation tasks should use JDK 1.6
+project.disableDeprecatedJvmTargetWarning()
+tasks
+    .matching { it.name == "compileKotlin" && it is UsesKotlinJavaToolchain }
+    .configureEach {
+        (this as UsesKotlinJavaToolchain).kotlinJavaToolchain.toolchain.use(project.getToolchainLauncherFor(JdkMajorVersion.JDK_1_6))
+    }
+
+tasks
+    .matching { it.name == "compileJava" && it is JavaCompile }
+    .configureEach {
+        (this as JavaCompile).javaCompiler.set(project.getToolchainCompilerFor(JdkMajorVersion.JDK_1_6))
+    }
+
 dependencies {
     compileOnly(project(":core:util.runtime"))
     compileOnly(project(":core:descriptors"))
@@ -23,19 +37,6 @@ sourceSets {
     "test" { projectDefault() }
 }
 
-val compileJava by tasks.getting(JavaCompile::class) {
-    sourceCompatibility = "1.6"
-    targetCompatibility = "1.6"
-}
-
-val compileKotlin by tasks.getting(KotlinCompile::class) {
-    kotlinOptions {
-        jvmTarget = "1.6"
-        jdkHome = rootProject.extra["JDK_16"] as String
-        freeCompilerArgs += "-Xsuppress-deprecated-jvm-target-warning"
-    }
-}
-
 val generateTests by generator("org.jetbrains.kotlin.generators.tests.GenerateRuntimeDescriptorTestsKt")
 
 projectTest(parallel = true) {