Add milestones to work items with mock query

What does this MR do and why?

This MR adds mock queries for milestones widget in work items.

Screenshots or screen recordings

Screenshots	Screenshots

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the invite modal

   Feature.enable(:work_items_mvc_2)

2. Visit any work item i.e a task
3. See that you can see the list if milestones that can be associated with the work item
4. Since this is only a mock , the selected milestone will not be associated with the work item always , that will be a separate milestone

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 #367463 (closed)