Fix ActivityPub context not being dynamically computed (#11746)

* Fix contexts not being dynamically included

Fixes #11649

* Refactor Note context in serializer

* Refactor Actor serializer

spec/lib/activitypub/activity/update_spec.rb

@@ -19,7 +19,7 @@ RSpec.describe ActivityPub::Activity::Update do
   end
 
   let(:actor_json) do
-    ActiveModelSerializers::SerializableResource.new(modified_sender, serializer: ActivityPub::ActorSerializer, key_transform: :camel_lower).as_json
+    ActiveModelSerializers::SerializableResource.new(modified_sender, serializer: ActivityPub::ActorSerializer, adapter: ActivityPub::Adapter).as_json
   end
 
   let(:json) do