Array declaration issue

I have fixed the declaration issue. Please have a look!

The patch is not correct, it changes implementation's signature directly without any research.

The interface in core Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface::mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem have changed to current signature `public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button, HTMLRestrictions $text_format_html_restrictions): ?array` which is the same implemented by the module.

The interface was changed on core 9.4 and 10.0.x and backported to 9.3, check this comment here → .

The signature displayed on the error message dates back to 9.3 initial release, check this commit.

Also cannot reproduce the error using the issue with the proposed versions.