Change system_time_zones() to use \Drupal\Core\Datetime\TimeZoneFormHelper::getOptionsList()

Problem/Motivation

• Refactor system_time_zones() to use \Drupal\Core\Datetime\TimeZoneFormHelper::getOptionsList() since after Drupal 10.1.x, we need to fix the following warning at k8s_timezone_allowed_values_function():
  

  system_time_zones() is deprecated in drupal:10.1.0 and is removed from drupal:11.0.0. This function is no longer used in Drupal core. Use \Drupal\Core\Datetime\TimeZoneFormHelper::getOptionsList(), \Drupal\Core\Datetime\TimeZoneFormHelper::getOptionsListByRegion() or \DateTimeZone::listIdentifiers() instead. See https://www.drupal.org/node/3023528
      25x in K8sCloudConfigTest::testCloudConfig from Drupal\Tests\k8s\Functional\cloud\config
      9x in K8sCloudConfigTest::testK8sMenu from Drupal\Tests\k8s\Functional\cloud\config
      4x in K8sCloudConfigTest::testCloudConfigRedirect from Drupal\Tests\k8s\Functional\cloud\config
      1x in K8sCloudConfigTest::testCloudConfigBulk from Drupal\Tests\k8s\Functional\cloud\config