Do not run flutter upgrade (#418)

.github/workflows/android.yaml

@@ -36,7 +36,6 @@ jobs:
         python3 collect_translations.py
     - name: Build for Android
       run: |
-        flutter upgrade
         flutter pub get
         cp lib/dummy_dsn.dart lib/dsn.dart
         flutter build apk --debug

.github/workflows/ci.yaml

@@ -46,7 +46,6 @@ jobs:
           python3 collect_translations.py
       - name: Static Analysis Tests
         run: |
-          flutter upgrade
           cp lib/dummy_dsn.dart lib/dsn.dart
           python3 find_dart_files.py
           flutter pub get

.github/workflows/ios.yaml

@@ -33,7 +33,6 @@ jobs:
         python3 collect_translations.py
     - name: Build for iOS
       run: |
-        flutter upgrade
         flutter pub get
         cd ios
         pod repo update