Apply using cweagans/composer-patches, patch it for package magento/module-theme!

fix_varnish_missing_topmenu_github_3421.patch

--- view/frontend/layout/default.orig.xml	2020-06-10 09:03:29.889000000 +0200
+++ view/frontend/layout/default.xml	    2020-06-10 09:03:36.624645526 +0200
@@ -70,7 +70,7 @@
                     <arguments>
                         <argument name="title" translate="true" xsi:type="string">Menu</argument>
                     </arguments>
-                    <block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="Magento_Theme::html/topmenu.phtml" ttl="3600" before="-"/>
+                    <block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="Magento_Theme::html/topmenu.phtml" before="-"/>
                 </block>
                 <block class="Magento\Framework\View\Element\Text" name="store.links" group="navigation-sections">
                     <arguments>