Use parent/child relationship when creating a task

What does this MR do and why?

Instead of just relating work items (original work item and newly created task) in "workItemCreateFromTask" mutation, parent/child relationship is created between them. Originally it was planned to support both types of relationships but because Task will rely on always having a parent (because of navigation), we will always create parent/child relationship only - #365130 (comment 1025718706)

Related to #365130 (closed)

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How to set up and validate locally

Feature.enable(:work_items)
Feature.enable(:work_items_hierarchy)

mutation {
  workItemCreateFromTask(
    input: {
      id: "gid://gitlab/WorkItem/576",
      workItemData: {
        lineNumberStart: 1,
        lineNumberEnd: 2,
        title: "New work item title",
        workItemTypeId: "gid://gitlab/WorkItems::Type/5",
        lockVersion: 6
      }
    }
  ) {
    errors
    workItem {
      id
      title
      description
    }
    newWorkItem {
      id
      title
      description
      workItemType {
        name
        id
      }
    }
  }
}

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