Resolve "Replace `data-confirm` modals with `GlModal` in `app/views/admin/hooks/edit.html.haml`"

What does this MR do and why?

Update the confirmation button variant to danger and its label to Delete on delete system hook confirmation modal.

Screenshots or screen recordings

Without FF	With FF (before)	With FF (after)

How to set up and validate locally

1. Enable the bootstrap_confirmation_modals feature flag: Feature.enable(:bootstrap_confirmation_modals)
2. Login as admin
3. Go to http://localhost:3000/admin/hooks
4. If there isn't a system hook, create one by filling the form and clicking Add system hook
5. Click 'Edit' next to one of the system hooks on the list
6. Click Delete

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 #344162 (closed)