[terraform] Rename ecs task stop alarm to denote staging or prod

Summary:
I noticed that I didn't include whether an ecs task stop alarm was staging or prod when I recently got an email for one.

Although these alarms should always be checked, it's helpful to know the severity of the alarm as well as whether to check staging or prod when these alarms trigger.

Test Plan: terraform plan successfully showed name change for staging ecs task stop alarm.

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13272