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/whenNullableSmartCast10.kt

@@ -1,4 +1,5 @@
-// LANGUAGE_VERSION: 1.0
+// !API_VERSION: 1.0
+
 fun box(): String {
     val nullValue: Any? = null
     val nullDouble: Double? = null
@@ -26,5 +27,6 @@ fun box(): String {
     }
     return "OK"
 }
+
 // 4 areEqual \(Ljava/lang/Object;Ljava/lang/Object;\)Z
 // 4 areEqual