The recent migrations of build.ros.org and build.ros2.org have brought HTTPS support to both build farms. As a result of the url scheme change web hooks for automatic pull request building must be updated.
If the GitHub machine user account ros-pull-request-builder has the
Admin permission on your ROS repositories then a new https hook has likely been added to your repositories already and the only recommended action is to remove the previous http hook which is no longer being delivered.
If you created the hook manually using the instructions in the wiki you will need to update the webhook to use HTTPS before pull request triggers will work.