updating flutter version in GitHub Actions CI pipeline

Author: saksham-gt

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ on:
   workflow_dispatch:
 
 env:
-  flutter_version: "2.10.x"
+  flutter_version: "3.0.x"
   flutter_channel: "stable"
   java_version: "12.x"
 

test/application/user_details/user_details_fixtures.dart

@@ -9,7 +9,7 @@ final IUserRepository tUserRepo = MockUserRepository();
 
 // ignore: avoid_positional_boolean_parameters
 Future<String?> testGetAnonymousTokenId([bool forceRefresh = false]) =>
-    Future.value(null);
+    Future.value();
 
 const User tUser = User(
   id: 'tId',