fix/simplify internal help method hopExecutorIfAtCfDelayerThread 👁️ 🧬

- fix wrong visibility modifier, `public` -> `private`
- simplify generic type declare, `CompletionStage` -> `CompletableFuture`

cffu-core/src/main/java/io/foldright/cffu/CompletableFutureUtils.java

@@ -1645,8 +1645,7 @@ C completeOnTimeout(C cf, @Nullable T value, long timeout, TimeUnit unit) {
     }
 
     @SuppressWarnings("unchecked")
-    public static <C extends CompletionStage<?>>
-    C hopExecutorIfAtCfDelayerThread(C cf, Executor asyncExecutor) {
+    private static <C extends CompletableFuture<?>> C hopExecutorIfAtCfDelayerThread(C cf, Executor asyncExecutor) {
         final CompletionStage<Object> f = (CompletionStage<Object>) cf;
 
         return (C) f.handle((v, ex) -> null).thenCompose(unused -> {