Steps to reproduce:

1. Clean installation of IBEXA DXP 3.3
2. Create a new content type with image and check the "can be a thumbnail" option
3. Generate migration file using command line:

   php bin/console ibexa:migrations:generate --type=content_type --mode=update
   

Result:
The thumbnail option is not included in the migration file

Expected Result:
In the fields of our content type with image should be

thumbnail: true