Adding ARR impact for availability issues

Why is this change being made?

In the Infrastructure/Quality offsite, we talked about the fact that a single customer that drives significant ARR for GitLab having an outage could constitute a severity1 incident. I've picked greater than $1mm/arr as an example of bias to action but don't have any strong feelings about what that number should be.

The severitization based on ARR impact is a function of GitLab having paying customers and a recognition that this has always been the case. There are concerns that this will be difficult to determine for folks who are on call. This MR is intended to be a place to have that discussion. We also discussed the fact that this is another branch for folks dealing with an incident to assess.

Consideration should be given to how folks discover this during an active incident. As always, I'm happy for this MR to be closed if there's a better way to represent the goal - to acknowledge in incident assessment the importance to GitLab as an organization of customers driving significant revenue. Thought has and should be given to the fact that other severitization factors weren't changed here and therefore this doesn't change GitLab's historic approach to severitization intentionally.

We may want to review the benchmark for different severities with other departments as well.

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