Migrate repo to use JVM toolchains Gradle feature.

^KT-46972 Fixed

libraries/stdlib/jvm-minimal-for-test/build.gradle.kts

@@ -7,8 +7,7 @@ plugins {
     `maven-publish`
 }
 
-jvmTarget = "1.6"
-javaHome = rootProject.extra["JDK_16"] as String
+project.configureJvmToolchain(JdkMajorVersion.JDK_1_6)
 
 val builtins by configurations.creating {
     isCanBeResolved = true
@@ -54,11 +53,6 @@ val copySources by task<Sync> {
     into(File(buildDir, "src"))
 }
 
-tasks.withType<JavaCompile> {
-    sourceCompatibility = "1.6"
-    targetCompatibility = "1.6"
-}
-
 tasks.withType<KotlinCompile> {
     dependsOn(copySources)
     kotlinOptions {