Add description_html to `PipelineSecurityReportFinding` GraphQL type

What does this MR do and why?

This MR adds the description_html to the PipelineSecurityReportFinding GraphQL type. This is to allow the pipeline to access this field via GraphQL query!

Screenshots or screen recordings

How to set up and validate locally

1. Run the following query for a Pipeline:

query {
  project(fullPath:"<project path>") {
    pipeline(iid:"<pipeline IID>") {
      securityReportFindings(first:1) {
        nodes {
          descriptionHtml
        }
      }
    }
  }
}

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 #368228 (closed)