Make spinners in app/views/admin/users/_users.html.haml Pajamas-compliant

What does this MR do and why?

Updates the spinner with new Pajamas code.

Screenshots or screen recordings

Scenario	Before	After
Light theme
Dark theme

How to set up and validate locally

1. Visit Admin > Users. For example, http://127.0.0.1:3000/admin/users.
2. If you want to persist the loader (stop the app from loading), apply this diff:

diff --git a/app/assets/javascripts/pages/admin/users/index.js b/app/assets/javascripts/pages/admin/users/index.js
index 41e99a3baf5..3ed636387ae 100644
--- a/app/assets/javascripts/pages/admin/users/index.js
+++ b/app/assets/javascripts/pages/admin/users/index.js
@@ -1,7 +1,7 @@
 import { initAdminUsersApp, initDeleteUserModals, initAdminUserActions } from '~/admin/users';
 import initConfirmModal from '~/confirm_modal';
 
-initAdminUsersApp();
+// initAdminUsersApp();
 initAdminUserActions();
 initDeleteUserModals();
 initConfirmModal();

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.

Related to #353763 (closed)