Minor ActivityPub JSON fixes (#4214)

- Objects must have attributedTo instead of actor
- The current attribute belongs to CollectionPage, not Collection

app/presenters/activitypub/collection_presenter.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 class ActivityPub::CollectionPresenter < ActiveModelSerializers::Model
-  attributes :id, :type, :current, :size, :items
+  attributes :id, :type, :size, :items
 end