Enable paging for GA

[BigLep]

Done Criteria

1. The FOC WG has a paging tool they're onboarded with
2. sev2.x alerts cut a page (rather than relying exclusively on slack)

Why Important

When we're GA, it's important that we treat sev2.x events as higher priority than everything else and requiring immediately engagement.

Notes

1. We deferred this for M4.1 as we work on baking in our alarms / thresholds.
2. We need to investigate/decide between using BetterStack vs. FF PagerDuty etc.
3. We should ideally be timezone sensitive. Lets page people who are most likely already awake.
4. Ideally we have a way to differentiate between sev2 (page instantly) and sev2.5 (page during business hours)