ISTWCMS-4957: Remove use of deprecated ThemeHandler::install()

https://www.drupal.org/node/3017233

uw_sites_all.install

@@ -27,8 +27,7 @@ function uw_sites_all_install() {
   }
 
   // Make sure the theme is installed.
-  $theme_handler = \Drupal::service('theme_handler');
-  $theme_handler->install(['uw_theme_admin']);
+  \Drupal::service('theme_installer')->install(['uw_theme_admin']);
 
   // Set it as the admin theme.
   $config = \Drupal::service('config.factory')->getEditable('system.theme');