zendesk/zendesk_api_client_php is abandoned

Problem/Motivation

Zendesk has abandoned their zendesk/zendesk_api_client_php package, see https://github.com/zendesk/zendesk_api_client_php and https://packagist.org/packages/zendesk/zendesk_api_client_php

However, in their API documentation they still link to that library, see https://developer.zendesk.com/documentation/ticketing/api-clients/php/

Proposed resolution

We need to remove that dependency, but since there is presumably no replacement, I don't know exactly what we should do about it.