Replace `window.confirm` in Vue/JS code with `GlModal` confirmation

What does this MR do and why?

To prepare migration steps for this epic &6993 (closed) I've picked one of the issues to walk through the solution. Basically, it replaces window.confirm call with the implementation that uses GlModal to confirm user action in app/assets/javascripts/diffs/components/diff_line_note_form.vue. Here is the MR that implements that confirmation - !73167 (merged).

Screenshots or screen recordings

Before	After

How to set up and validate locally

Open some merge request => add review comment to it=> Cancel => You should see a confirmation that uses GlModal instead of browser confirm modal.

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 #344118 (closed)