Migrate `form_errors` in create user form to Pajamas alert

What does this MR do and why?

Migrate form_errors in app/views/admin/users/_form.html.haml to Pajamas::AlertComponent

Screenshots or screen recordings

Scenario	Screenshot
Before
After

How to set up and validate locally

1. With an admin account, go to http://localhost:3000/admin/users
2. Click on New user.
3. In the Name, Username, and Email field enter the value s and press Create user
4. Alert should be shown with error message.

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 #364962 (closed)