use page header description to show the subtitle

app/views/activities/index.html.erb

@@ -32,16 +32,12 @@ See COPYRIGHT and LICENSE files for more details.
 <%=
   render Primer::OpenProject::PageHeader.new do |header|
     header.with_title { (@author.nil? ? t(:label_activity) : t(:label_user_activity, value: link_to_user(@author))).html_safe }
+    header.with_description { t(:label_date_from_to, start: format_date(@date_to - @days), end: format_date(@date_to-1)) }
     header.with_breadcrumbs([{ href: home_path, text: organization_name},
                              *([href: project_overview_path(@project.id), text: @project.name] if @project),
                              t(:label_activity)])
   end
 %>
-<%=
-  render(Primer::OpenProject::SubHeader.new) do |subheader|
-    subheader.with_text {t(:label_date_from_to, start: format_date(@date_to - @days), end: format_date(@date_to-1))}
-  end
-%>
 
 <%= render(Activities::DaysComponent.new(events: @events, current_project: @project, activity_page: @activity_page))  %>