Switch to GlModal for user confirmation dialogs

Related issue - #344169 (closed)

What does this MR do and why?

Switch to GlModal for confirmation dialogs in the admin user section.

Screenshots or screen recordings

Disable 2FA

before	after

Secondary email

before	after

How to set up and validate locally

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.