[RN][iOS] Fix codegen imports

[cipolleschi]

With the most recent of React Native, we renamed the React-Codegen pod to ReactCodegen.
We are testing react-native-exit-app in our CI against react-native nightlies to make sure that we don't break libraries. You can find the latest execution here: https://github.com/facebook/react-native/actions/runs/14633368975/job/41059664787

Test Plan

1. Run this script to create a new app with a nightly

npx @react-native-community/cli init MyApp --version nightly --skip-install
yarn add react-native-exit-app
cd ios
bundle install
bundle exec pod install
cd ..
yarn ios

2. Observe the app failing to build

3. Apply the changes in this PR in the node_modules/react-native-exit-app/ folder

4. Run yarn ios again

5. observe the app build successfully.