Issue #3436741 make Automatic Updates Extension stable

Closes #3436741

automatic_updates_extensions/src/Form/UpdateReady.php

@@ -30,7 +30,7 @@ use Symfony\Component\EventDispatcher\EventDispatcherInterface;
  * Defines a form to commit staged updates.
  *
  * @internal
- *   Form classes are internal.
+ *   Form classes are internal and should not be used by external code.
  */
 final class UpdateReady extends UpdateFormBase {
 
@@ -133,7 +133,11 @@ final class UpdateReady extends UpdateFormBase {
     $form['package_updates'] = $this->showUpdates();
     $form['backup'] = [
       '#prefix' => '<strong>',
-      '#markup' => $this->t('Back up your database and site before you continue. <a href=":backup_url">Learn how</a>.', [':backup_url' => 'https://www.drupal.org/node/22281']),
+      '#type' => 'checkbox',
+      '#title' => $this->t('Warning: Updating contributed modules or themes may leave your site inoperable or looking wrong.'),
+      '#description' => $this->t('Back up your database and site before you continue. <a href=":backup_url">Learn how</a>. Each contributed module or theme may follow different standards for backwards compatibility, may or may not have test, and may add or remove features in any release. For this reason, it is highly recommended that you test this update in a development environment first.', [':backup_url' => 'https://www.drupal.org/node/22281']),
+      '#required' => TRUE,
+      '#default_value' => FALSE,
       '#suffix' => '</strong>',
     ];
     $form['maintenance_mode'] = [