Change convert type to work item MVC feature flag

What does this MR do and why?

MR introducing it: !112221 (merged). We decided that it's not necessary to have its own feature flag, so we're moving it to the work_item_mvc_2 feature flag.

• ✅ Feature Flag does not exist on production or staging

Screenshots or screen recordings

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How to set up and validate locally

1. Feature.enable(:work_items_mvc_2)
2. Create a work item (e.g. OKR, Key Result, or Task)
3. Fill in the data and change the types
4. You can get the work item type IDs by quering: WorkItems::Type.all

query workItemTypes {
  group(fullPath: "gitlab-org") {
    workItemTypes {
      nodes {
        id
        name
      }
    }
  }
}

query workItems {
  group(fullPath: "gitlab-org") {
    issues {
      edges {
        node {
          id
        }
      }
    }
  }
}

mutation convertWorkItem {
  workItemConvert(
    input: {id: "gid://gitlab/WorkItem/643", workItemTypeId: "gid://gitlab/WorkItems::Type/2"}
  ) {
    clientMutationId
    workItem {
      id
      workItemType {
        id
        name
      }
    }
    errors
  }
}

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.