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/tools/stdlib-compiler-classpath/build.gradle.kts

@@ -0,0 +1,9 @@
+plugins {
+    java
+    id("jps-compatible")
+}
+
+sourceSets {
+    "main" { projectDefault() }
+    "test" {}
+}