Fix an error when adding a new rule on Drupal 9 fails with SchemaIncompleteException

Drupal\Core\Config\Schema\SchemaIncompleteException: Entity type 'Drupal\Core\Config\Entity\ConfigEntityType' is missing 'config_export' definition in its annotation in Drupal\Core\Config\Entity\ConfigEntityBase->toArray() (line 251 of core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php).
Drupal\Core\Config\Entity\ConfigEntityStorage->mapToStorageRecord(Object) (Line: 283)
Drupal\Core\Config\Entity\ConfigEntityStorage->doSave('case_study', Object) (Line: 452)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 263)
Drupal\Core\Config\Entity\ConfigEntityStorage->save(Object) (Line: 339)
Drupal\Core\Entity\EntityBase->save() (Line: 591)
Drupal\Core\Config\Entity\ConfigEntityBase->save() (Line: 297)
Drupal\menu_position\Form\MenuPositionRuleForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 113)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 592)
Drupal\Core\Form\FormBuilder->processForm('menu_position_rule_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 705)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Looks like it's not currently covered by Upgrade Status: https://www.drupal.org/project/upgrade_status/issues/3131063

And may be related to this change record: https://www.drupal.org/node/2481909