Replace transfer group jquery with GL components

What does this MR do and why?

Removes some unnecessary jquery from the group transfer form, as well as implements some enhancements as per #347189 (comment 797186951)

• Replace jQuery dropdown with gl-dropdown equivalent
• Update button type from warning to danger
• Add confirmation modal when clicking the transfer group button

Screenshots or screen recordings

Initial state	Paid group	Namespace dropdown open	Group / Namespace selected	Confirmation modal

How to set up and validate locally

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

1. Navigate to group settings http://<gdk>/groups/<group>/-/edit
2. Expand the "advanced" settings
3. Select a namespace from the list
4. Click "Transfer group"
5. Confirm the action in the confirmation modal
6. The page will refresh with the new path in the url

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 #347189 (closed)