Issue #2137217 by aspilicious: Remove install_time() variable get usage.

core/modules/system/system.install

@@ -277,7 +277,7 @@ function system_requirements($phase) {
     // Determine when cron last ran.
     $cron_last = \Drupal::state()->get('system.cron_last');
     if (!is_numeric($cron_last)) {
-      $cron_last = variable_get('install_time', 0);
+      $cron_last = \Drupal::state()->get('install_time', 0);
     }
 
     // Determine severity based on time since cron last ran.