Improve runbook for disabling a Sidekiq queue

In production#3875 (closed) we decided to disable a specific queue update_namespace_statistics:namespaces_root_statistics. To do this, we decided to remove the queue from the selector list in https://ops.gitlab.net/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/merge_requests/4. Because of an issue where there was a dependency on charts.gitlab.io delivery#1596 (closed) we decided to edit the configmap directly.

After editing the Chart and cycling the pod it looked like the queue was still processing jobs.

• Understand why the direct edit of the configmap didn't work for disabling the queue
• Ensure we have a good runbook for disabling a queue