Replace `data-confirm` modals with `GlModal` in `app/views/projects/artifacts/_artifact.html.haml`

What does this MR do and why?

Related to #344179 (closed) as part of &6854 (closed).

The confirm modal works as expected for the "🗑 [Artifact]" button. And this MR:

• Adds the data-confirm-btn-variant="danger" attribute to the button.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:bootstrap_confirmation_modals)

2. Optionally enable the artifacts management page

   Feature.enable(:artifacts_management_page)

3. Visit http://127.0.0.1:3000/gitlab-org/gitlab-test/-/artifacts. You might need to run a job first.
4. Click the "🗑" button.
5. The confirm modal should show "Delete artifacts", and it'd delete it.

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.