CRON monitor alerts in Slack should show resolutions

### Problem Statement

When I read a Slack notification about a CRON monitor that timedout or was missed, I can't know the current status of this issue. Was it resolved or is this still a problem?

It's especially interesting when you only look at the Slack channel a few times a day and you have multiple CRONs that can fail. Solving this would help narrowing the down the important alerts, those that still need to be solved.

### Solution Brainstorm

The Slack integration should probably update the previous notification or post in the same thread, to notify that the CRON issue is resolved.

Sentry already published manual resolutions and reopening in the Slack thread, see:

### Product Area

Crons, Alerts

Start by locating the Slack integration for CRON monitor alerts and the existing manual resolution and reopening behavior referenced in the issue. Trace how timeout and missed notifications are posted, then determine how a resolved status should appear in the original message or thread. Done means Slack recipients can identify when a CRON alert has been resolved.

Written by the indexing model from the issue text.