Ajax callback is not callable

Problem/Motivation

While selecting a province or changing other fields, the ajax callback fails with an error which states that the callback is not callable.

Steps to reproduce

1. Add the field to an entity.
2. Try creating an entity with the field.
3. Select a province.
4. The locality field will not populate and you will be able to see an error in the console.

Proposed resolution

The fix is to change the way the ajax callback is defined on the widget to align with latest Drupal standards.

Remaining tasks

Test this fix on the sites affected.

User interface changes

None.

API changes

None.

Data model changes

None.