Drupal 11 support

I removed Drupal 9/10 in core_version_requirement because I removed everything related to the editor.image_dialog route (which was removed in Drupal 11), but we might want to publish a transition release compatible with both Drupal 10 and Drupal 11 first.

phpstan is complaining that it does not find ckeditor (4) classes.
I think the solution would be to add drupal/ckeditor to dev dependencies in composer.json.
But do we want to keep supporting CKE 4? The ckeditor contrib module is abandoned.