Deprecate AnnotatedClassDiscovery

📌 Triggering deprecations for plugins using annotations when core plugin type has been converted to attributes Needs work does both, trigger deprecations for plugins still using annotations and plugin managers not providing an attribute class. But it only does so for plugin managers that use DefaultPluginManager and don't customize the getDiscovery() method. I guess it makes sense to also explicitly deprecate using the class through other means. Will result in two deprecations in most cases, but that's OK.