Add button to access new vulnerability form

What does this MR do and why?

Create a new button which links the user to the new vulnerability form. This button is only visible when the :new_vulnerability_form is enabled and user is on the project vulnerability page.

You'll need to enable the feature flag to see the button:

Feature.enable(:new_vulnerability_form)

Screenshots or screen recordings

Other dashboards remain unchanged - here's a screenshot from the group dashboard:

How to set up and validate locally

1. Enable :new_vulnerability_form feature flag
2. Visit Project > Security & Compliance > Vulnerability Report

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #301005 (closed)