Context:
With the introduction of event monitoring we have a name conflict between dto's relating to the event monitoring and legacy dto's that are named `Event` but actually relate to an bpmn activity.

We should consistently rename dto's that relate to activities to contain `FlowNode` instead of `Event` or `Activity`.

AT:

• relevant dto's are renamed accordingly
• fields of dtos like e.g. ProcessInstanceDto are renamed
  • index mappings are migrated accordingly