Expose solution on VulnerabilityType

What does this MR do and why?

Expose solution on VulnerabilityType. This will allow vulnerability solutions to be retrieved via GraphQL.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. If your GDK does not have vulnerabilities, run bundle exec rake 'gitlab:seed:vulnerabilities'

2. Go to <instance_url>/-/graphql-explorer

3. Run this query:

   {
     vulnerability(id: "gid://gitlab/Vulnerability/1") {
       id
       description
       solution
     }
   }

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.