[Task]: Healthcheck should notify of recovery

[KevinMind]

Description

As of #15440 the healtcheck slack notification is only when there are failures.

When the healthcheck workflow runs, if the previous run was a failure and the current run was a success, we should get a notification that the issue is resolved.

Acceptance Criteria

Milestones/checkpoints

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Checks

• If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

┆Issue is synchronized with this Jira Task

[diox]

Note: We now have a dummy monitor that depends on a waffle switch to play with

[diox]

With the way things are implemented right now, the recovery is easier to implement by just running the healthcheck more often (like once per hour) so that we don't have to bother with recording state.

[diox]

We are putting this on the back burner until other notifications-related bits are done. Ultimately both failures and recoveries should not be that frequent...