Add solutionHtml field to SecurityReportFinding

What does this MR do and why?

This MR adds the solutionHtml field to the PipelineSecurityReportFindingType.

Screenshots or screen recordings

How to set up and validate locally

1. Run the query on a set of security findings that has some solutions, some not (https://gitlab.com/gitlab-examples/security/security-reports):

query {
  project(fullPath:"root/security-reports") {
    pipeline(iid:"<pipeline IID>") {
      securityReportFindings(first:10) {
        nodes {
          title
          reportType
          solution
          solutionHtml
        }
      }
    }
  }
}

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 #384510 (closed)