Remove Notification Subscriptions

Regenerate the private URL key used to construct a user's personal iCal calendar feed URLs (`http_cal_url` and `web_cal_url`). The old key is immediately invalidated; callers should update any stored feed subscriptions with the new key. A user may only regenerate their own key. Account admins may regenerate on behalf of any user. The calendar feed URLs act as bearer credentials: anyone with the URL can read that user's on-call calendar without further authentication. Rotate the key here whenever a URL may have been exposed (for example, if it was shared outside the intended audience or if an admin with `can_update_user` retrieved it via `GET /users/{id}?include[]=calendar_urls`). Users are members of a PagerDuty account that have the ability to interact with Incidents and other data on the account. For more information see the [API Concepts Document](https://developer.pagerduty.com/api-reference/a47605517c19a-api-concepts#users) Scoped OAuth requires: `users.write`