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/jvm/build.gradle

@@ -19,6 +19,7 @@ dependencies {
     api(kotlinStdlib())
     testApi(project(":kotlin-test:kotlin-test-junit"))
     testApi('junit:junit:4.13.2')
+    kotlinCompilerClasspath(project(':libraries:tools:stdlib-compiler-classpath'))
 }
 
 jar {