Add cancel button

What does this MR do and why?

Adds a cancel button the new vulnerability form. This button leads the user back to the Vulnerability Report page.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. You'll need an EE license.
2. Navigate to :namespace/:project/-/security/vulnerabilities/new using the Address bar
3. Hit cancel button, it should bring you back to the vulnerability report page

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 #301003 (closed)