Prohibit JVM target 1.6

But still compile stdlib, reflect, kotlin.test and scripting runtimes
with JVM target 1.6 to simplify migration from Kotlin 1.6 to 1.7.

 #KT-45165 Fixed

core/util.runtime/build.gradle.kts

@@ -7,6 +7,7 @@ project.configureJvmToolchain(JdkMajorVersion.JDK_1_6)
 
 dependencies {
     compileOnly(kotlinStdlib())
+    kotlinCompilerClasspath(project(":libraries:tools:stdlib-compiler-classpath"))
 }
 
 sourceSets {