Use API_VERSION instead of LANGUAGE_VERSION in IEEE754 tests

See `ExpressionCodegen.genEqualsForExpressionsPreferIeee754Arithmetic`:
the behavior here actually depends on the API version, not any language
feature

compiler/testData/codegen/bytecodeText/ieee754/when10.kt

@@ -1,4 +1,5 @@
-// LANGUAGE_VERSION: 1.0
+// !API_VERSION: 1.0
+
 fun box(): String {
     val plusZero: Any = 0.0
     val minusZero: Any = -0.0
@@ -29,4 +30,4 @@ fun box(): String {
 }
 
 // 4 areEqual \(Ljava/lang/Object;Ljava/lang/Object;\)Z
-// 4 areEqual
+// 4 areEqual