Migrate to shared alert HAML partial in _gitaly_unavailable.html.haml

app/views/shared/errors/_gitaly_unavailable.html.haml

-.gl-alert.gl-alert-danger.gl-mb-5.gl-mt-5
-  .gl-alert-container
-    = sprite_icon('error', size: 16, css_class: 'gl-icon gl-alert-icon gl-alert-icon-no-title')
-    .gl-alert-content
-      .gl-alert-title
-        = reason
+= render 'shared/global_alert',
+  alert_class: 'gl-my-5',
+  variant: :danger,
+  dismissible: false,
+  title: reason do
   .gl-alert-body
     = s_('The git server, Gitaly, is not available at this time. Please contact your administrator.')