Add projects widget to instance security dashboard

What does this MR do?

This MR adds a /-/security/vulnerable_projects/ endpoint that fetches the projects on a user's instance security dashboard that have vulnerabilities. It includes the count for each vulnerability severity in the response.

Note: the target of this MR is the extract-vulnerable-projects-finder branch until that branch has been merged. Once it's merged I'll change the target and remove WIP, but I want to get this MR reviewed now to lessen the risk it slips out of %12.8.

Issue: #17969 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation ([if required] !24652 (merged)(https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-required))
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team