Change link for upgrades and renewals link

See this issue for context: https://gitlab.com/gitlab-com/sales/issues/300 - Sales team handles upgrades and renewal so we are changing this link from a Support form to the Sales form.

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

@@ -7,7 +7,7 @@
     .billing-plans-alert.card.prepend-top-10
       .card-header.bg-warning.text-white
         = s_("BillingPlans|Automatic downgrade and upgrade to some plans is currently not available.")
-        - customer_support_url = 'https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=334447';
+        - customer_support_url = 'https://about.gitlab.com/sales/';
         - customer_support_link = link_to s_("BillingPlans|Customer Support"), customer_support_url
         = s_("BillingPlans|Please contact %{customer_support_link} in that case.").html_safe % { customer_support_link: customer_support_link }