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

compiler/config.jvm/src/org/jetbrains/kotlin/config/JvmTarget.kt

@@ -55,5 +55,8 @@ enum class JvmTarget(
             return if (platformDescription != null) "JVM target $platformDescription"
             else "JVM bytecode version $majorVersion"
         }
+
+        fun supportedValues(): List<JvmTarget> =
+            values().asList() - JVM_1_6
     }
 }