Re-add additional bottom margin to error summary content

Reverses a visual change introduced in #4971 where the negative space at the bottom of the Error Summary component was reduced to equalise the container padding on all sides.

Although this was originally approved of by a designer on the team, further discussion after the 5.4.0 release concluded that having additional negative space below the Error Summary's content was desirable, as it gave extra space for text descenders to occupy (if present).

packages/govuk-frontend/src/govuk/components/error-summary/_index.scss

@@ -30,6 +30,10 @@
     > * + * {
       @include govuk-responsive-margin(4, "top");
     }
+
+    > :last-child {
+      @include govuk-responsive-margin(1, "bottom");
+    }
   }
 
   // Cross-component class - adjusts styling of list component