Resolve "Add Severity Overrides to Vulnerability Report API Calls"

What does this MR do and why?

This merge request adds support for displaying severity overrides for vulnerabilities. It introduces a new SeverityOverrideEntity to represent severity override information, updates the Vulnerability model to include a latest_severity_override association, and modifies existing entities and helpers to include this new data. The changes allow for tracking and displaying when a vulnerability's severity has been manually changed, including details like the original severity, new severity, who made the change, and when. The API and frontend components are updated to include this new information in vulnerability details, and tests are added to ensure the new functionality works as expected.

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Related to #517446 (closed)