Support older version of Turbo for confirm modal

`Turbo.config` was introduced in version 8.0.6[^1]. This change ensures that
apps using older versions of Turbo can still get the custom confirmation modal
behaviour and not error out.

[^1]: https://github.com/hotwired/turbo/releases/tag/8.0.6

Author: forkata

admin/app/javascript/solidus_admin/application.js

@@ -4,4 +4,8 @@ import "solidus_admin/controllers"
 import { openConfirmModal } from "solidus_admin/confirm_modal"
 import "solidus_admin/web_components/solidus_select"
 
-Turbo.config.forms.confirm = openConfirmModal
+if (Turbo.config) {
+  Turbo.config.forms.confirm = openConfirmModal
+} else {
+  window.Turbo.setConfirmMethod(openConfirmModal)
+}