Replace window.confirm with GlModal for board setting sidebar

What does this MR do and why?

This MR solves: #344119 (closed) it replaces the browser confirm modal with GlModal.

Screenshots or screen recordings

before	after
before-board	remove-list-button

How to set up and validate locally

1. go to http://gitlab.localdev:3000/$GROUP/$PROJECT/-/boards
2. on a project that has some Issues
3. click create List
4. create a custom list
5. click on setting of that custom list
6. in the sidebar click the remove list button
7. see the modal in action

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

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.