Use the backend theme when upgrading

Related core PR:
civicrm/civicrm-core#25961

This change was initially introduced in CiviCRM 4.2 to avoid
issues when upgrading, but we could not find any reason to keep
it that way today. Using the backend theme will make it easier
to provide consistent theming accross CMS-es and also usually
avoids some frontend CMS features to kick-in, such as Views.

modules/civicrmtheme/src/Theme/CivicrmThemeNegotiator.php

@@ -67,10 +67,6 @@ public function applies(RouteMatchInterface $route_match) {
       return FALSE;
     }
 
-    if (count($parts) > 1 && $parts[1] == 'upgrade') {
-      return FALSE;
-    }
-
     $config = $this->configFactory->get('civicrmtheme.settings');
     $admin_theme = $config->get('admin_theme');
     $public_theme = $config->get('public_theme');