Drop support for DKImagePickerController

[navaronbracke]

The dependency DKImagePickerController is only used when picking media in resolveMultiPickFromGallery().

https://github.com/search?q=repo%3Amiguelpruivo%2Fflutter_file_picker%20DKImagePickerController&type=code

However, maintenance on that third-party dependency is very minimal as of today, which would prevent us from adding the Privacy Manifest for #1418

We should remove that third-party dependency and use Apple vendored API's to reimplement the method.
Then we can unblock the work for the Privacy Manifests, which has a hard deadline in two months.

cc @jamesdixon

[navaronbracke]

DKImagePickerController did provide a release with a PrivacyManifest. Since we do not pin the version, updating your Pods is sufficient.