Clarify keyword arguments with ** (#12769)

This change is to suppress the warning below on on ruby-2.7.0:

- warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call

https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

app/lib/activitypub/activity.rb

@@ -21,7 +21,7 @@ class ActivityPub::Activity
   class << self
     def factory(json, account, **options)
       @json = json
-      klass&.new(json, account, options)
+      klass&.new(json, account, **options)
     end
 
     private