Fix REST/GRAPHQL APIs handling TODOs WorkItem target

What does this MR do and why?

Fixes a bug when a target_type of a TODO is WorkItem instead of Issue. This affected both APIs

How to set up and validate locally

1. Create a TODO for the logged in user by mentioning the user in an Issue note, leave it as pending
2. In the console find that todo and change the target_type to WorkItem (todo.update!(target_type: 'WorkItem'))
3. curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/todos"
4. You should get an error before this change (uninitialized constant API::Entities::WorkItem)
5. In GraphiQL make a query like:

{
  currentUser {
    todos {
      nodes {
        id
	targetType
      }
    }
  }
}

It should also fail before this change

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 #374954 (closed)