Drupal 9 compatibility

This patch only applied to main module not to the child ones.
Please review again.

Adding core_version_requirement: ^8 || ^9 to submodules

I am getting two errors in Drupal 9.0.6 related to the Flags module. It may be issues of deprecated code: the first one related to https://www.drupal.org/project/flags/issues/3170231.

Also, from Flags UI:

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('am', 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: 171)
Drupal\flags_ui\Form\ConfigEntityFormBase->save(Array, Object) (Line: 76)
Drupal\flags_ui\Form\CountryMappingForm->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: 593)
Drupal\Core\Form\FormBuilder->processForm('country_flag_mapping_add_form', Array, Object) (Line: 321)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object) (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->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: 50)
Drupal\ban\BanMiddleware->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)