[#71059] Adjust sprint form error spacing

Let the flex row own the vertical spacing so the danger banner aligns
with the other sprint form rows.

https://community.openproject.org/wp/71059

Author: myabc

modules/backlogs/app/components/backlogs/sprint_form_component.html.erb

@@ -41,7 +41,7 @@ See COPYRIGHT and LICENSE files for more details.
       flex_layout(mb: 2) do |flex|
         if base_errors&.any?
           flex.with_row(mb: 3) do
-            render(Primer::Alpha::Banner.new(mb: 3, scheme: :danger)) { base_errors.join("\n") }
+            render(Primer::Alpha::Banner.new(scheme: :danger)) { base_errors.join("\n") }
           end
         end
         if shared_sprint?