Use new trial registration URL in billing

changelogs/unreleased/35751-new-trial-flow-for-logged-in-user-should-not-take-them-to-the-custo.yml

+---
+title: Use new trial registration URL in billing
+merge_request: 19978
+author:
+type: fixed

ee/app/views/shared/billings/_billing_plan_header.html.haml

@@ -31,4 +31,4 @@
     = render 'shared/billings/trial_status', namespace: namespace
 
   - if namespace.eligible_for_trial?
-    %p= link_to 'Start your free trial', new_gitlab_com_trial_url, class: 'btn btn-primary'
+    %p= link_to 'Start your free trial', new_trial_registration_path, class: 'btn btn-primary'