Minor, fix some codegen tests for language version 1.4

These tests check behavior of an old language version on purpose: the
original bug KT-24708 has been fixed by introducing an error here in
1.4.

compiler/testData/codegen/box/when/enumOptimization/differentEnumClasses.kt

@@ -1,4 +1,6 @@
+// !LANGUAGE: -ProhibitComparisonOfIncompatibleEnums
 // IGNORE_BACKEND_FIR: JVM_IR
+
 enum class A {
     O, K
 }

compiler/testData/codegen/box/when/enumOptimization/differentEnumClasses2.kt

@@ -1,4 +1,6 @@
+// !LANGUAGE: -ProhibitComparisonOfIncompatibleEnums
 // IGNORE_BACKEND_FIR: JVM_IR
+
 enum class A {
     OK
 }