Add solution section

What does this MR do and why?

This is the last part of the new vulnerability form. It allows adding an optional solution text to the vulnerability.

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to /-/security/vulnerabilities/new through the URL (should be a project scope)
2. The page should render the solution section
3. These fields will do nothing, that functionality will be added on subsequent MRs

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)