Expose uses_legacy_iid flag to test reports GraphQL type

What does this MR do and why?

Exposes uses_legacy_iid to the test report type so it can be used in the frontend.

Note: !108520 (merged) must be merged first.

How to set up and validate locally

Entertaining directions in !108520 (comment 1234984150) and the description of that MR. Instead of checking in console, use the following GraphQL query:

query {
  project(fullPath: "Flightjs/Flight") {
    requirements {
      nodes {
        iid
        workItemIid
        title
        testReports {
          nodes {
            id
            usesLegacyIid
          }
        }
      }
    }
  }
}

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 #382141 (closed)