Use 'JVM_TARGET: 1.8' directive in codegen tests on 1.8 target

compiler/testData/codegen/java8/compileKotlinAgainstKotlin/jvm8against6/simpleCallWithHierarchy.kt

@@ -7,7 +7,7 @@ interface Test {
 }
 
 // FILE: 2.kt
-// KOTLIN_CONFIGURATION_FLAGS: +JVM.JVM_8_TARGET
+// JVM_TARGET: 1.8
 interface Test2 : Test {
 
 }
@@ -19,4 +19,4 @@ class TestClass : Test2 {
 
 fun box(): String {
     return TestClass().test()
-}
+}