Attach or update the Slack user group for a schedule

curl --request PUT \
  --url https://api.shiftkeeper.io/schedules/{schedule_id}/slack_user_group/ \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "name": "Incident responders",
  "handle": "incident-responders"
}'

{
  "user_group": {
    "id": "S083J1KTL9X",
    "name": "Incident responders",
    "handle": "incident-responders"
  }
}