Waitlist validation subscriber better as swapped plugin class

Problem/Motivation

Working on 📌 Replace HostEntity::isEnabledForRegistration Active we realised that modules/registration_waitlist/src/EventSubscriber/RegistrationValidationEventSubscriber.php would be better implemented as new plugin class swapping out the core capacity plugin.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes