[Monitoring] Alerting action link is broken

[chrisronline]

For our out of the box alerts, we provide messaging that creates a deep-link back to the Stack Monitoring UI from the alert destination (email, slack, etc). This appears to be broken now and noticed in #83681

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[chrisronline]

Relates to #87344. We need to also change how we use Markdown