Expose legacy requirement IID in Work Items GraphQL API

What does this MR do and why?

Expose widget for legacy requirement with field legacy_iid, and immediately deprecate. This widget is to assist customers moving from Requirements API to Work Items API.

How to set up and validate locally

1. Create a new requirement under Issues -> Requirements in project sidebar

2. Use the following GraphQL query (fill in project full path) and get the IID and work item IID of one or more requirements:

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

3. Use the following GraphQL query with the work item IID from the above. Ensure the legacy IID is the same as the iid result above for each record.

query Requirement {
  workItem(id: "gid://gitlab/WorkItem/123") {
    title
    widgets {
      type
      ... on WorkItemWidgetRequirementLegacy {
        legacyIid
      }
    }
  }
}

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 #383016 (closed)