Fix Router Scaling Alarms

[JamieB-gu]

We've discovered that router is being scaled up via a LatencyNotificationAlarm rather than a LatencyScalingAlarm. We should stop using the LatencyNotificationAlarm for this and add a LatencyScalingAlarm instead.

Background

There are usually two latency alarms for each frontend stack:

• LatencyNotificationAlarm - for sending alerts to the team.
• LatencyScalingAlarm - for activating scaling policies.

For more context @JamieB-gu and @ioannakok can help.