Add workItemDelete mutation

What does this MR do and why?

Adds the ability to delete a work item via the GraphQL API. Authors of the work items can now delete work items even if not admins of the project.

Work Item Context

WorkItem model is another name for Issue. All part of the new work item initiative. Comment we related links for additional context on the initiative !76401 (comment 772061018)

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 {
  workItemDelete(input:{id: "gid://gitlab/WorkItem/1"}) {
    errors
    project {
      id
    }
  }
}

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 #346132 (closed)