Fix `Rails/WhereExists` cop in REST::TagSerializer model (#28790)

.rubocop_todo.yml

@@ -86,7 +86,6 @@ Rails/WhereExists:
     - 'app/models/status.rb'
     - 'app/policies/status_policy.rb'
     - 'app/serializers/rest/announcement_serializer.rb'
-    - 'app/serializers/rest/tag_serializer.rb'
     - 'app/services/activitypub/fetch_remote_status_service.rb'
     - 'app/services/vote_service.rb'
     - 'app/validators/reaction_validator.rb'

app/serializers/rest/tag_serializer.rb

@@ -19,7 +19,7 @@ class REST::TagSerializer < ActiveModel::Serializer
     if instance_options && instance_options[:relationships]
       instance_options[:relationships].following_map[object.id] || false
     else
-      TagFollow.where(tag_id: object.id, account_id: current_user.account_id).exists?
+      TagFollow.exists?(tag_id: object.id, account_id: current_user.account_id)
     end
   end