Moved milestones widget to mvc FF

What does this MR do and why?

This MR moves the milestones widget from :work_items_mvc_2 to :work_items_mvc

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

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

1. Enable the feature flag

   Feature.enable(:work_items_mvc)

2. Visit any work item page such as http://localhost:3000/flightjs/Flight/-/work_items/59

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.