Replace YAML approver info with persisted data

What does this MR do and why?

Replace YAML approver info with persisted data in order to make this information available for the frontend .

This will allow for the removal of a similar method which is currently limited to this controller.

Related issue: #357301 (closed)

Frontend MR: !90521 (merged)

How to set up and validate locally

1. Go to Security & Compliance -> Policies and create a scan result policies as per our docs with at least one approver.
2. Go to graphql-explorer and query the scan result policies with approver information:

{
  project(fullPath: <PROJECT_FULL_PATH>){
    scanResultPolicies{
      nodes{
        name
        userApprovers {
          id
	  webUrl
        }
        groupApprovers {
          id
          webUrl
        }
      }
    }
  }
}

MR acceptance checklist

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