[firebase_ui_auth] Allow latest facebook_flutter_auth

This fixes #470 by allowing a version of the dependency which adds support for Facebook Limited Login

Author: aebrahim

packages/firebase_ui_oauth_facebook/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
   firebase_ui_oauth: ^2.0.0
   flutter:
     sdk: flutter
-  flutter_facebook_auth: ^6.0.3
+  flutter_facebook_auth: ">=6.0.3 < 8.0.0"
 
 dev_dependencies:
   flutter_test: