Bump google_mobile_ads from 3.1.0 to 4.0.0 in /game_template (#2105)

Bumps
[google_mobile_ads](https://github.com/googleads/googleads-mobile-flutter/tree/main/packages)
from 3.1.0 to 4.0.0.
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game_template/pubspec.yaml

@@ -26,7 +26,7 @@ dependencies:
   firebase_core: ^2.1.1  # Needed for Crashlytics below
   firebase_crashlytics: ^3.3.5  # Error reporting
   games_services: ^4.0.0  # Achievements and leaderboards
-  google_mobile_ads: ^3.0.0  # Ads
+  google_mobile_ads: ^4.0.0  # Ads
   in_app_purchase: ^3.0.1  # In-app purchases
 
 dev_dependencies: