v5.2 Alert color is disabled

Created by: cccabinet

The alert setting was changed to a CSS variable in #35401, but the alert color is disabled.

ｖ5.1.3（https://getbootstrap.com/docs/5.1/components/alerts/#examples）

ｖ5.2.0（https://deploy-preview-35401--twbs-bootstrap.netlify.app/docs/5.1/components/alerts/#examples）

Please correct it.