DEV: Move about_stat_groups to DiscoursePluginRegistry (#20496)

Follow up to 098ab29d4150a5a39880d59ef6cad5f6795a18fd. Since
we just used a `cattr_reader` on `About` this was not safe
for multisite, since some sites could have the chat plugin
enabled and some may not. Using `DiscoursePluginRegistry` gets
around this issue, and makes it so the chat stats only show
for a site if `chat_enabled` is true.

lib/discourse_plugin_registry.rb

@@ -108,6 +108,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :search_groups_set_query_callbacks
 
+  define_filtered_register :about_stat_groups
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end