Add new field webUrl to VulnerabilityType

What does this MR do and why?

Adds a new field webUrl to the Vulnerability GraphQL response. This will help move the logic of building the URL from the frontend to the backend.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab/-/issues/381133+

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How to set up and validate locally

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MR acceptance checklist

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