Refactor milestone select to GlDropdown

What does this MR do and why?

This MR refactors the milestone selects on the issue and merge request /new and /edit pages from the old select2 dropdowns to Pajamas GlDropdown components, and also deletes milestone_select.js 🎉

Screenshots or screen recordings

before	after

How to set up and validate locally

Test the milestone dropdown on the issue and merge request /new and /edit pages:

• http://127.0.0.1:3000/h5bp/html5-boilerplate/-/issues/new
• http://127.0.0.1:3000/h5bp/html5-boilerplate/-/issues/1/edit
• http://127.0.0.1:3000/h5bp/html5-boilerplate/-/merge_requests/new
• http://127.0.0.1:3000/h5bp/html5-boilerplate/-/merge_requests/1/edit

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.