Bundled Themes: Twenty Twenty submenu items disappear underneath the Cover block.

Fixes the submenu issue by raising the z-index value of the site-header from 1 to 2.

Props kjellr, JavierCasares.
Fixes #49322.
Built from https://develop.svn.wordpress.org/trunk@47609


git-svn-id: https://core.svn.wordpress.org/trunk@47384 1a063a9b-81f0-0310-95a4-ce76da25c4cd