Fix XCode download link (#129795)

<img width="919" alt="image" src="https://github.com/flutter/flutter/assets/42003129/4bc8c3b8-29af-4b98-9232-47a583523e3c">
I was installing Flutter and noticed that the download link for Xcode is incorrect. They might have changed it, but the current link doesn't exist.

packages/flutter_tools/lib/src/base/user_messages.dart

@@ -183,10 +183,10 @@ class UserMessages {
       '  sudo xcodebuild -runFirstLaunch';
   String get xcodeMissing =>
       'Xcode not installed; this is necessary for iOS and macOS development.\n'
-      'Download at https://developer.apple.com/xcode/download/.';
+      'Download at https://developer.apple.com/xcode/.';
   String get xcodeIncomplete =>
       'Xcode installation is incomplete; a full installation is necessary for iOS and macOS development.\n'
-      'Download at: https://developer.apple.com/xcode/download/\n'
+      'Download at: https://developer.apple.com/xcode/\n'
       'Or install Xcode via the App Store.\n'
       'Once installed, run:\n'
       '  sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer\n'