fix responsive layout for GCP form

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

@@ -6,9 +6,9 @@
 = render_gcp_signup_offer
 
 .row.prepend-top-default
-  .col-sm-3
+  .col-md-3
     = render 'sidebar'
-  .col-lg-9.js-toggle-container
+  .col-md-9.js-toggle-container
     %ul.nav-links.nav-tabs.gitlab-tabs.nav{ role: 'tablist' }
       %li.nav-item{ role: 'presentation' }
         %a.nav-link{ href: '#create-new-cluster-pane', id: 'create-new-cluster-tab', class: active_when(active_tab == 'new'), data: { toggle: 'tab' }, role: 'tab' }