Set product_name specifically in the header

Now this header will be used outside the context of ContentBlockManager,
we should set the product name speficially, rather than relying on the
helper method being overridden.

lib/engines/content_block_manager/app/views/shared/_header.html.erb

@@ -1,7 +1,7 @@
 <% environment = GovukPublishingComponents::AppHelpers::Environment.current_acceptance_environment %>
 
 <%= render "govuk_publishing_components/components/layout_header", {
-  product_name:,
+  product_name: ContentBlockManager.product_name,
   environment: environment,
   logo_link: content_block_manager.content_block_manager_root_path,
   navigation_items: [