Bump google_mobile_ads from 5.3.1 to 6.0.0 in /game_template (#2605)

Bumps [google_mobile_ads](https://github.com/googleads/googleads-mobile-flutter/tree/main/packages) from 5.3.1 to 6.0.0.
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Author: dependabot[bot]

game_template/pubspec.yaml

@@ -26,7 +26,7 @@ dependencies:
   firebase_core: ^3.0.0  # Needed for Crashlytics below
   firebase_crashlytics: ^4.0.0  # Error reporting
   games_services: ^4.0.0  # Achievements and leaderboards
-  google_mobile_ads: ^5.0.0  # Ads
+  google_mobile_ads: ^6.0.0  # Ads
   in_app_purchase: ^3.0.1  # In-app purchases
 
 dev_dependencies: