Turn the Sentry Blackbox notifications into an asynchronous notification

As an EOC, the Sentry service blackbox notifications should not page since there is no easy action to take to fix Sentry. Investigate if we can either:

1. Create a slack-only version
2. Disable the blackbox notifications for sentry

• Example pagerduty incident: https://gitlab.pagerduty.com/incidents/Q0EWIMP9HDUWVK?utm_source=slack&utm_campaign=channel
• The Blackbox Rules: https://gitlab.com/gitlab-com/runbooks/-/blob/master/rules/blackbox_alerts.yml