feat(incident management): add new root cause

Why is this change being made?

Add a new root cause RootCause::FalseAlarm, we have certain incidents created by pages/alerts that aren't actionable and we don't have a good way to track this, one of the most recent examples is gitlab-com/gl-infra/production#6136 (closed)

Kudos to @igorwwwwwwwwwwwwwwwwwwww for suggesting the idea!

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