### Release note

Resolves elastic#158172

Fixes an issue where alerting on Synthetics monitors could sometimes
become delayed.

## Summary
Adjust alerting timing to account for alerting framework delays.

Alert executors do not fire perfectly on the specified interval (in our
case, every 1 minute). They are typically delayed a few seconds, be can
especially delayed if the customer does not have their alerting settings
configured to scale and there is no CPU available to take on the
scheduled task.

This PR accounts for small delays in the alerting framework, by
subtracting an additional 1 minute from the `from` value for our current
status query.

(cherry picked from commit d549500)