Remove mock from milestone widget

What does this MR do and why?

This MR removes the mock from milestone widgets and adds the capability of associating real milestones with a work item. Also , whenever a work item is created the milestone of the parent ( in this case issue ) is automatically associated with the work item

Screenshots or screen recordings

add_milestone

How to set up and validate locally

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

1. Enable the work_items_mvc2 feature flag

   Feature.enable(:work_items_mvc_2)

2. Open any issue assign it to a milestone
3. Create a task from the issue
4. When you open the task , you can see the parent milestone being associated with the task/child work item
5. Now hover over the milestone and override the milestone
6. Close the modal or task and open again to see that the manual override still persists.

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)