ci: upgrade upload-artifact action to v4

- Update 'actions/upload-artifact' action version from v3 to v4 for workflow
  optimization and compatibility.

Author: muzahidul-opti

.github/workflows/flutter.yml

@@ -300,7 +300,7 @@ jobs:
           fi
 
       - name: Upload APK artifacts
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: minification-test-apk
           path: |