Raise 'kotlin.jvm.target.validation.mode' default value to 'error'.

Now target check intentionally runs always - even when Java sources are
empty. Java part configures publication 'org.gradle.jvm.version'
attribute which affect  published artifact consumers and should be the
same as Kotlin jvmTarget value.

^KT-54993 Fixed

libraries/stdlib/jdk8/build.gradle

@@ -110,6 +110,11 @@ task testJdk6Tests(type: Test) { thisTask ->
     })
 }
 
+tasks.named("compileModuleTestJava", JavaCompile) {
+    sourceCompatibility = JavaVersion.VERSION_1_9
+    targetCompatibility = JavaVersion.VERSION_1_9
+}
+
 compileModuleTestKotlin {
     kotlinJavaToolchain.toolchain.use(JvmToolchain.getToolchainLauncherFor(project, JdkMajorVersion.JDK_9_0))
 }