Issue #3466655: Add D11 support

• Removed implementation of ContainerAwareInterface and ContainerAwareTrait.
• Updated constructor to accept ConfigFactoryInterface as a dependency.
• Added a static create method to instantiate the class with the required service.
• Updated service definition in services.yml to pass the config.factory service.
• Ensured the get method uses the injected configFactory service.

This change improves code maintainability and testability by explicitly injecting dependencies.

Related to #3466655

modules/flexmail_webform/flexmail_webform.info.yml

 name: 'Flexmail Webform'
 type: module
 description: 'Flexmail email service webform integration'
-core_version_requirement: ^9 || ^10
+core_version_requirement: ^9 || ^10 || ^11
 package: 'Flexmail'
 dependencies:
   - webform:webform