Align infradev severity matrix definition to include Dedicated

Why is this change being made?

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Impact of bugs on Dedicated is distinct in nomenclature and threshold than on GitLab.com. Work has already completed to harmonize the severity matrix definition for Incidents, under the Incident 2.0 initiative: https://handbook.gitlab.com/handbook/engineering/infrastructure/incident-management/#incident-severity.

The impact on availability of the timely prioritization and resolution of infradev issues is high on the customer experience AND on being able to onboard new customers. The prioritization is dependent on the severity matrix that drives infradev issue prioritization, especially for availability: https://handbook.gitlab.com/handbook/product-development/how-we-work/issue-triage/#availability.

This MR attempts to align the two critical severity definitions and improve prioritization of issues that impact Dedicated product Availability and SaaS scale.

There are two critical changes:

1. Use "% of user impact" for GitLab.com and "tenant" for Dedicated offering when sizing impact.
2. Add repeated incidents as a source of higher severity, as this impacts the ability of Dedicated offering to scale.

Author and Reviewer Checklist

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• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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Commits

• Added Dedicated impact statement to Availability severity table to distinguish from GitLab.com wording

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