Add Detected Vulnerabilities SLOs

Why is this change being made?

As part of this FY22-Q3 KR, this MR introduce new SLOs for Detected Vulnerabilities. The goal is to avoid having them aging in Vulnerability Reports without any warning or notification.

We have many Vulnerability Reports to triage, and sometimes Detected Vulnerabilities are waiting for a long time before getting attention. Some of them can have a Critical severity, and don't necessary appear on our radar before browsing all these reports.

The GitLab Inventory will be used to spot violations of these SLOs, because we need to differentiate projects based on their categories.

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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refs !91396