Skip processing Update activities on unknown accounts (#15514)

This also skips fetching the actor completely.

This will be useful if we end up distributing Update activities linked to
account suspensions more widely (they are currently only delivered to
the suspended account's followers), as currently, instances not knowing
about the suspended account would fetch it to then process the suspension.

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/controllers/activitypub/inboxes_controller.rb

@@ -5,7 +5,7 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
   include JsonLdHelper
   include AccountOwnedConcern
 
-  before_action :skip_unknown_actor_delete
+  before_action :skip_unknown_actor_activity
   before_action :require_signature!
   skip_before_action :authenticate_user!
 
@@ -18,13 +18,13 @@ class ActivityPub::InboxesController < ActivityPub::BaseController
 
   private
 
-  def skip_unknown_actor_delete
-    head 202 if unknown_deleted_account?
+  def skip_unknown_actor_activity
+    head 202 if unknown_affected_account?
   end
 
-  def unknown_deleted_account?
+  def unknown_affected_account?
     json = Oj.load(body, mode: :strict)
-    json.is_a?(Hash) && json['type'] == 'Delete' && json['actor'].present? && json['actor'] == value_or_id(json['object']) && !Account.where(uri: json['actor']).exists?
+    json.is_a?(Hash) && %w(Delete Update).include?(json['type']) && json['actor'].present? && json['actor'] == value_or_id(json['object']) && !Account.where(uri: json['actor']).exists?
   rescue Oj::ParseError
     false
   end