Heiwa: Add site-footer-container css class to remove the gap between the main and the footer (#6001)

heiwa/templates/header-footer-only.html

@@ -4,4 +4,4 @@
 <main class="wp-block-group" style="margin-top:0px;margin-bottom:0px"><!-- wp:post-content {"layout":{"inherit":true}} /--></main>
 <!-- /wp:group -->
 
-<!-- wp:template-part {"slug":"footer","tagName":"footer"} /-->
+<!-- wp:template-part {"slug":"footer","tagName":"footer","className":"site-footer-container"} /-->