Create vulnerabilities route for instance-level

What does this MR do?

Create vulnerabilities route for instance-level

• similar to how groups/dashboard and groups/vulnerabilities is separated, now there is a security/dashboard and security/vulnerabilities
• add tests
• this route is not accessible in this MR; that MR is to come

Screenshots

No new UI changes, but in the future it will look like the following using this new route

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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

Related to #230412 (closed)