Registration creation service leaks data

Problem/Motivation

The RegistrationCreationService is a stateful object which stores data internally. Since the service is not declared as a non-shared service, it is leaking its state to any code that retrieves the service from the container.

Steps to reproduce

// Run some code that sets an event instance on the service (e.g. `EventInstanceCapacity::render()`).
// ...

// Now in some later running code retrieve the service.
$service = \Drupal::service('recurring_events_registration.creation_service')
// We should have a clean instance of the service, but the data from the previous code is still present.
assert(empty($service->getEventInstance()), 'No data is leaked between uses of the service');

Proposed resolution

Mark the service as non-shared.