Apply 'alertdialog' ARIA role to DangerDialog

Danger dialogs (both the confirmation and default, warning variant)
clearly adhere to the alert dialog pattern:

> The alertdialog role is to be used on modal alert dialogs that
> interrupt a user's workflow to communicate an important message and
> require a response.

See: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/alertdialog_role

Author: myabc

app/components/primer/open_project/danger_dialog.rb

@@ -82,12 +82,14 @@ def initialize(
         @confirm_button_text = confirm_button_text
         @cancel_button_text = cancel_button_text
 
-        @system_arguments = system_arguments
+        @system_arguments = deny_tag_argument(**system_arguments)
+        deny_single_argument(:role, "`role` will always be set to `alertdialog.", **@system_arguments)
         @system_arguments[:id] = @dialog_id
         @system_arguments[:classes] = class_names(
           system_arguments[:classes],
           "DangerDialog"
         )
+        @system_arguments[:role] = "alertdialog"
 
         @dialog = Primer::Alpha::Dialog.new(title: title, subtitle: nil, visually_hide_title: true, **@system_arguments)
       end