Refactor severity vulnerabilities count to support group- and instance- reports

What does this MR do?

It refactors and prepares both the <vulnerability-count-list /> component and the vulnerabilitySeveritiesCount query to support the scoping of vulnerability-severity counts to either a project, group or instance.

This will allow the severity count to be added to the group- and instance- level vulnerability reports, which will happen in separate MRs:

MR	description
👉 (current)	refactor to support scoping of severity counts
!47863 (merged)	add count to group level vulnerability report
!47861 (merged)	add count to instance level vulnerability report

How to test this

1. Go to the "Vulnerability Report" page for one of your projects. (eg.: https://gitlab.com/gitlab-org/gitlab/-/security/vulnerability_report).
2. There should be no changes between this branch and master.

Screenshots (strongly suggested)

before	after

No visible changes should be introduced.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - no visible changes
• [-] Documentation (if 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

Related to #262109 (closed)