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/smartCastsForFloat10.kt

@@ -1,4 +1,4 @@
-// LANGUAGE_VERSION: 1.0
+// !API_VERSION: 1.0
 
 fun equals5(a: Any?, b: Any?) = if (a is Float && b is Float?) a == b else null!!
 
@@ -20,4 +20,4 @@ fun box(): String {
 }
 
 // 3 areEqual \(Ljava/lang/Object;Ljava/lang/Object;\)Z
-// 3 areEqual
+// 3 areEqual