Add severity override fragment

What does this MR do and why?

This merge request adds support for severity overrides in the vulnerability data structure. The GraphQL fragment for vulnerabilities now includes a new field for severity overrides, allowing the system to fetch and display custom severity levels for vulnerabilities. Additionally, the test mock data has been updated to include an empty array for severity overrides in various vulnerability objects. These changes likely enable users to manually adjust the severity of vulnerabilities, providing more flexibility in how security issues are prioritized and managed within the system.

References

Related to #510979 (closed)

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