Use Executor::CreateSerial

PiperOrigin-RevId: 316809187

Author: wu-hui

firestore/src/common/settings_ios.mm

@@ -19,8 +19,7 @@
 
 Settings::Settings()
     : host_(kDefaultHost),
-      executor_(absl::make_unique<ExecutorLibdispatch>(dispatch_queue_create(
-          "com.google.firebase.firestore.callback", DISPATCH_QUEUE_SERIAL))) {}
+      executor_(Executor::CreateSerial("com.google.firebase.firestore.callback")) {}
 
 std::unique_ptr<Executor> Settings::CreateExecutor() const {
   return absl::make_unique<ExecutorLibdispatch>(dispatch_queue());