Migrate reduce visibility confirmation modal to gl-modal

What does this MR do and why?

This MR updates the "reduce project visibility" confirmation modal, migrating from the HAML implementation to vue and making some small visual tweaks to align closer to the rest of our existing confirmation modals.

Related to #344561 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Update the method body of show_visibility_confirm_modal?

   def show_visibility_confirm_modal?(project)
+    return true
     project.unlink_forks_upon_visibility_decrease_enabled? && project.visibility_level > Gitlab::VisibilityLevel::PRIVATE && project.forks
   end

1. Create a project with public visibility
2. Navigate to project settings for your new project
3. Change the visibility to private
4. Click save

MR acceptance checklist

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