clean_maintenance-8.x-1.3/clean_maintenance.install

<?php

/**
 * @file
 * The installation file.
 */

/**
 * Implements hook_install().
 */
function clean_maintenance_install() {

  // Set the default message.
  $message = "@site is currently under maintenance. We should be back shortly. Thank you for your patience.";

  // Set the site name.
  \Drupal::configFactory()->getEditable('system.maintenance')->set('message', $message)->save();
}