Fix variant handling in metatag_page_manager

I'm not sure if this is the same issue or not, but as far as I can tell metatag_page_manager isn't working (in the sense of "I never see my configured metatags on my page manager page").

The metatag_page_manager_metatag_route_entity method looks for the variant as a routeMatch parameter.

if ($variant = $route_match->getParameter('page_manager_page_variant')) {

At least in the work I am doing on D10.3 setting up a page with a couple of variants on a new path, I never see that parameter set. Instead I have to look at defaults:

$variant_id = $route_match->getRouteObject()->getDefault('_page_manager_page_variant')

My knowledge of the way Page Manager loads Pages and PageVariants is extremely superficial, so this may not be smart or secure, but with this change I do see my metatags loading on my page instances.

@floydm this is a new bug. MR and patch can be found here 🐛 Update Page Manager integration for latest version Active