Ajax callback function is un-callable
They way you specify your callback means that it is a function. You define it in your class, which means, it is not.

Prefix it with ::, that's a shortcut that tells Drupal that it's a method on the form class. You should also use camel case for the method's names. So, something like ::ajaxExampleDependentDropdownCallback (to be clear, you also have to rename your method, not just the callback string).

I also don't understand what you're trying to do with include 'dependent_selects.config';?