Considering that you have done all the steps I would follow, I would suggest:
Check your dependencies
It may be an error with your dependencies. Some versions may still 'force' your system to run <13.0.
You may look in your flutter terminal. Run 'flutter pub get' and check the outdated dependencies. Or you may visit the dependency on flutter website and check the latest version.
Clean Xcode cache
flutter clean
flutter pub get
cd ios
xcodebuild clean