refactor(firebase_dart_flutter): upgrade sign_in_with_apple to ^6.0.0

Author: rbellens

packages/firebase_dart_flutter/pubspec.yaml

@@ -17,7 +17,7 @@ dependencies:
   package_info: ^2.0.0
   google_sign_in: ^5.0.0
   flutter_facebook_auth: ^4.3.4+2
-  sign_in_with_apple: ^4.0.0
+  sign_in_with_apple: ^6.0.0
   url_launcher: ^6.0.0
   logging: ^1.0.2
   uni_links_desktop: ^0.1.3