Fix the ios_fullscreen app

[loic-sharma]

Fixes the ios_fullscreen add-to-app sample app:

1. The Flutter module was renamed from flutter_module to flutter_module_fullscreen. However, the iOS native app had not been updated to reflect this
2. Flutter's minimum deployment version was raised to 13.0. The iOS native app targeted iOS 12.4.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[loic-sharma]

A bunch of changes here were done by Xcode when I ran the app 🤷

Manual changes in this file:

1. Replaced flutter_module with flutter_module_fullscreen
2. Added IPHONEOS_DEPLOYMENT_TARGET = 13.0; to raise minimum deployment target to meet Flutter's requirements.