Rework RootCause labels to cover more of our incident triggers

Why is this change being made?

Motivation:

Many of our incidents don't fit the current set of RootCause labels. This MR proposes some incremental changes in order to cover more ground and clarify the distinction between expected vs unexpected increases in demand.

Summary:

• Renamed RootCause::Malicious-Traffic to RootCause::External-Traffic to include bursts in legit traffic.
• Renamed RootCause::Saturation to RootCause::Capacity and clarify that it applies to cases where we expected to keep up with demand.
• Introduced RootCause::False-Positive to track incidents that were non-issues or non-actionable.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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