Fix missing validation on `default_privacy` setting (#25513)

app/models/user_settings.rb

@@ -14,7 +14,7 @@ class UserSettings
   setting :show_application, default: true
   setting :default_language, default: nil
   setting :default_sensitive, default: false
-  setting :default_privacy, default: nil
+  setting :default_privacy, default: nil, in: %w(public unlisted private)
 
   namespace :web do
     setting :crop_images, default: true