ci: use 3.x over empty string (#22)

Author: alestiago

.github/workflows/very_good_test_runner.yaml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         flutter_version:
           - "3.10.0" # The Flutter version with a Dart SDK that matches the minimum Dart SDK version supported by the package.
-          - "" # The latest stable version of Flutter.
+          - "3.x" # The Flutter version with a Dart SDK that matches the maximum Dart SDK version supported by the package.
 
     uses: VeryGoodOpenSource/very_good_workflows/.github/workflows/flutter_package.yml@v1
     with: