chore(api): upgrade to api-platform 3.0

[usu]

ToDo

• check performance timings (sanity check before/after)
• refactor/fix tests for state providers and processors
• fix phpstan/psalm warning
• review deprecation warnings

Stuff for separate PRs

• Review changes done in https://github.com/api-platform/api-platform and take over what makes sense
• Our two DTOs Invitation and ResetPassword use slightly different concepts on how to return data. Review best practices for DTO in api-platform v3 and implement consistently for both DTOs.
• Review if we want to support multiple ApiResources in our various extensions (UriTemplateFactory, etc.)
• Register upstream issues

[usu]

I've implemented 2 examples for State Processors (=replacement of Data Persisters).

@ecampcore: Would be nice to review + discuss this latest in the next meeting. After that we could migrate the remaining data persisters.

Raw example (no abstraction; as per Api-Platform documentation)

ActivityCreateProcessor
ActivityRemoveProcessor

Abstracted example (automatic split of before/after logic + property change listeners)

This is is similar as we had it before. However we don't need the separation between various operations and custom actions anymore, as we can now define a separate processor per action out of the box.

CampCollaborationCreateProcessor
CampCollaborationResendInvitationProcessor
CampCollaborationUpdateProcessor