WorkItemUpdate mutation allows to update title

What does this MR do and why?

Allows the WorkItemUpdate mutation to update the title attribute

How to set up and validate locally

1. Run bundle exec rails console
2. Run Feature.enable(:work_items)
3. Execute a GraphQL query like

mutation {
  workItemUpdate(input: { id: "gid://gitlab/WorkItem/1", title: "updated title" }) {
    workItem {
      id
      title
    }
  }
}

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 #343668 (closed)