Migrate to shared alert in admin/groups/_form.html

What does this MR do and why?

Migrate to shared alert in app/views/admin/groups/_form.html.haml.

It also fixes the previous alert not having a variant (displayed without background color - screenshots below).

Part of &5213.

Screenshots or screen recordings

before	after

How to set up and validate locally

• Login as admin
• Navigate to admin area > groups
• Click "New group" button
• Scroll down to the end of the page and observe the info alert

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 #325353 (closed)