flutter/flutter#67598 and flutter/flutter#67781 introduces a new flutter build ipa command.
Reference in the iOS deployment docs.

1. Remove the flutter build ios instructions.
2. Replace

1. Select Product > Archive to produce a build archive
2. In the sidebar of the Xcode Organizer window, select your iOS app, then select the build archive you just produced.

with something like:

1. Run flutter build ipa
2. Open the archive inbuild/ios/archive in Xcode by running open -a Xcode build/ios/archive/YourApp.xcarchive or locating it in Finder and opening it.

Keep the "Click the Validate App button." next step.

Add something about flutter build ipa with hints for how to populate the optional --export-options-plist file.