Extensions can use the methods onSkinTemplateNavigation::Universal and onSidebarBeforeOutput to modify menus in all skins except Minerva.
In Minerva, a MobileMenu hook is provided to allow modifications to the menu.
It's proposed that we drop the MobileMenu hook and allow extensions to use those hooks to make modifications instead.
# Acceptance criteria
[] Update DefaultMainMenuBuilder and AdvancedUserMenuBuilder to source personal menu items from SkinTemplateNavigation::Universal (user-menu group) https://gerrit.wikimedia.org/r/c/mediawiki/skins/MinervaNeue/+/753596
[] Update DefaultMainMenuBuilder and AdvancedUserMenuBuilder to source home from SkinTemplateNavigation::Universal (user-menu group)
[] Extension:GrowthExperiments should replace usages of MobileMenu with SkinTemplateNavigation::Universal
[] Hard deprecate MobileMenu hook