Replace `data-confirm` modals with `GlModal` in `app/views/shared/doorkeeper/applications/_delete_form.html.haml`

What does this MR do and why?

Related to #344206 (closed) as part of &6854 (closed).

The confirm modal works as expected for both variants of the "Destroy [oauth application]" button. And this MR:

• Adds the data-confirm-btn-variant="danger" attribute to both buttons.
• Adds an aria-label to the big/normal button variant.

Screenshots or screen recordings

Before	Small variant	Big/normal variant

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:bootstrap_confirmation_modals)

2. Visit http://127.0.0.1:3000/admin/applications and create or view an application.
3. Click the "Destroy" or "🗑" button.
4. The confirm modal should show "Destroy" and it'd delete the application.

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.