Remaining fixes for Drupal 11 Compatiblity

Summary

Convert deprecated @FieldWidget annotations to PHP 8 #[FieldWidget] attributes for Drupal 11 compatibility. Annotation-based plugin discovery is removed in Drupal 11, so the widgets are not discovered
at runtime despite the info.yml declaring ^11 support.

Changes required

1. src/Plugin/Field/FieldWidget/SelectStringTextfieldWidget.php — Convert @FieldWidget annotation to #[FieldWidget] attribute
2. src/Plugin/Field/FieldWidget/SelectStringTextareaWidget.php — Convert @FieldWidget annotation to #[FieldWidget] attribute
3. src/Plugin/Field/FieldWidget/SelectFormattedTextfieldWidget.php — Convert @FieldWidget annotation to #[FieldWidget] attribute
4. src/Plugin/Field/FieldWidget/SelectFormattedTextareaWidget.php — Convert @FieldWidget annotation to #[FieldWidget] attribute
5. src/WidgetHelper.php — Replace list($key, $value) with [$key, $value] array destructuring (minor cleanup)

All four widget files need:

• Add use Drupal\Core\Field\Attribute\FieldWidget;
• Add use Drupal\Core\StringTranslation\TranslatableMarkup;
• Replace @FieldWidget(...) docblock with #[FieldWidget(...)] attribute
• Replace @Translation("...") with new TranslatableMarkup('...')

Safe for all supported versions — attribute discovery works in Drupal 10.2+, and core_version_requirement already requires ^10.3.