Update dart.yml

Author: cloudwebrtc

.github/workflows/dart.yml

@@ -22,7 +22,7 @@ jobs:
           java-version: '12.x'
       - uses: subosito/flutter-action@v1
         with:
-          flutter-version: '2.2.3'
+          flutter-version: '2.8.0'
           channel: 'stable'
       - run: flutter packages get
       - run: flutter format lib/ test/ --set-exit-if-changed