Swap some iOS package CODEOWNERS (flutter#5667)

Take back codeownership from @stuartmorgan.  See flutter/packages#3793.

Author: jmagman

CODEOWNERS

@@ -71,17 +71,17 @@ packages/video_player/video_player_android/**            @camsim99
 
 # - iOS
 packages/camera/camera_avfoundation/**                   @hellohuanlin
-packages/file_selector/file_selector_ios/**              @stuartmorgan
+packages/file_selector/file_selector_ios/**              @jmagman
 packages/google_maps_flutter/google_maps_flutter_ios/**  @hellohuanlin
 packages/google_sign_in/google_sign_in_ios/**            @vashworth
 packages/image_picker/image_picker_ios/**                @vashworth
 packages/in_app_purchase/in_app_purchase_storekit/**     @louisehsu
-packages/ios_platform_images/ios/**                      @stuartmorgan
+packages/ios_platform_images/ios/**                      @jmagman
 packages/local_auth/local_auth_ios/**                    @louisehsu
-packages/path_provider/path_provider_foundation/**       @stuartmorgan
+packages/path_provider/path_provider_foundation/**       @jmagman
 packages/quick_actions/quick_actions_ios/**              @hellohuanlin
 packages/shared_preferences/shared_preferences_foundation/**  @tarrinneal
-packages/url_launcher/url_launcher_ios/**                @stuartmorgan
+packages/url_launcher/url_launcher_ios/**                @jmagman
 packages/video_player/video_player_avfoundation/**       @hellohuanlin
 packages/webview_flutter/webview_flutter_wkwebview/**    @hellohuanlin