Migrate "Purchase storage" button in alert

What does this MR do and why?

Migrates the "Purchase storage" button in alert to confirm variant.

Before	After

MR acceptance checklist

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ee/app/views/shared/_namespace_storage_limit_alert.html.haml

@@ -31,6 +31,6 @@
         - if show_storage_banner_actions
           .gl-alert-actions
             - if purchase_link
-              = link_to(_('Purchase more storage'), purchase_link, class: "btn gl-alert-action btn-md gl-button btn-success")
+              = link_to(_('Purchase more storage'), purchase_link, class: "btn gl-alert-action btn-md gl-button btn-confirm")
             - if usage_link
               = link_to(_('View usage details'), usage_link, class: "btn gl-alert-action btn-md gl-button btn-default")