Update Xcode minimum (#1246)

After the app store started requiring Xcode 16 in April 2025, Firebase now requires at least 16.2

Author: paulb777

firebaseai/testapp/readme.md

@@ -9,7 +9,7 @@ inside the Unity Editor.
 ## Requirements
 
 * [Unity](http://unity3d.com/) The quickstart project requires 2021.3 or higher.
-* [Xcode](https://developer.apple.com/xcode/) 15.1 or higher
+* [Xcode](https://developer.apple.com/xcode/) 16.2 or higher
   (when developing for iOS).
 * [Android SDK](https://developer.android.com/studio/index.html#downloads)
   (when developing for Android).