DEB-1483 keycloak: Fixed adult/child reg, refactored ApiResponse, better error logging, some 🧹

DEB-1483 keycloak: CiceroRegistrationUserCreation - fixed child/adult reg + some 🧹

DEB-1483 keycloak: ApiError - fixed toString (validationErrors were printed as object reference).

DEB-1483 keycloak: Using new ApiResponse all around - where needed + some 🧹-ing 😍

- Removed ApiFetchResult 🧹

DEB-1483 keycloak: CiceroApi - refactored code to use new ApiResponse, also removed partially implemented session key caching.

DEB-1483 keycloak: Added ApiResponse test for creation (of), added dependency convergence plugin and smaller version fixes.

DEB-1483 keycloak: refactored ApiResponse, added tests.

- enabled preview features - can be removed when upgrading to java 21 - as keylcoak allows for