build(dependencies): add override for core dependency

- Add dependency override for core package version 1.3.1
- Include TODO comment for future removal when newer version is released
- This change supports the removal of LocalAd functionality

Author: fulleni

pubspec.yaml

@@ -91,6 +91,8 @@ flutter:
   generate: true
 
 dependency_overrides:
+  # TODO: Remove this override when core version > 1.3.1 is released.
+  # This is needed for the removal of LocalAd.
   core:
     git:
       url: https://github.com/flutter-news-app-full-source-code/core.git