Migrate work item milestone dropdown to listbox

Screenshots or screen recordings

it shows old and new dropdown side by side (the left one is old where the right one is migrated)

How to set up and validate locally

Copied instructions from here

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable the feature flag Feature.enable(:linked_work_items)

Steps to verify:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page. Check the milestone dropdown - try to update it, make sure the value is persisted.

Related to #418086 (closed)