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

libraries/kotlin.test/junit/build.gradle

@@ -16,6 +16,7 @@ dependencies {
     expectedBy project(':kotlin-test:kotlin-test-annotations-common')
     api project(':kotlin-test:kotlin-test-jvm')
     api('junit:junit:4.13.2')
+    kotlinCompilerClasspath(project(':libraries:tools:stdlib-compiler-classpath'))
 }