Replace window.confirm with GlModal for noteable discussions

What does this MR do and why?

This MR solves: #344125 (closed) it replaces the browser confirm modal with GlModal.

Screenshots or screen recordings

before	after
cancel-comment-before	better-modal

How to set up and validate locally

1. open any MR
2. write a comment into it
3. start to reply to that comment
4. cancel the reply
5. see 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.