Use EventListeners

[stephanpelikan]

Since Camunda 8.6 EventListeners are available. Currently, events are consumed based on GRPC subscriptions. The downside is that the recently introduced zeebe-user tasks are not tracked neither cancellation of tasks (e.g. due to boundary events).

Check if EventListeners fulfill the need (Created, Cancelled) and if so, use them as the new default mechanism. If feasible the current mechanism should be available by settings a dedicated Spring property. Hint: Those event-listeners have to added not by the person modeling the BPMN but by the adapter during deployment.

See also vanillabp/business-cockpit#34.