Automatically alert RM when patch release threshold has been reached

Once the patch release threshold (to be defined on #2896 (closed)) has been reached, release managers need to be aware that a patch release should be prepared.

The patch release pressure is available on delivery-metrics gateway (#2690 (closed)), which should allow us to build a Prometheus alert that automatically verifies the patch release data and send an alert to #f_upcoming_release channel when the threshold is met.