Add class gl-alert-body for body

- changelog

app/views/projects/branches/new.html.haml

@@ -6,7 +6,8 @@
     = sprite_icon('error', size: 16, css_class: 'gl-icon gl-alert-icon gl-alert-icon-no-title')
     %button.js-close.gl-alert-dismiss{ type: 'button', 'aria-label' => _('Dismiss') }
       = sprite_icon('close', size: 16, css_class: 'gl-icon')
-    = @error
+    .gl-alert-body
+      = @error
 %h3.page-title
   New Branch
 %hr

changelogs/unreleased/233658-migrate-alert-on-new-branch-page.yml

+---
+title: Migrates the alert on the new branch page
+merge_request: 50307
+author:
+type: other