CKEditor4To5UpgradePluginInterface::mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem() was made more capable

\Drupal\ckeditor5\Plugin\CKEditor4To5UpgradePluginInterface::mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem() gained a new parameter.

Technically, this is a backwards compatibility break. But since this is still a very new API and we found zero real-world usages of this API, we went ahead with a (simple) BC break.

Before

  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button): ?string;

After

  public function mapCKEditor4ToolbarButtonToCKEditor5ToolbarItem(string $cke4_button, HTMLRestrictions $text_format_html_restrictions): ?array;

Because:

   * @param \Drupal\ckeditor5\HTMLRestrictions $text_format_html_restrictions
   *   The restrictions of the text format, if this upgrade plugin needs to
   *   inspect the text format's HTML restrictions to make a decision.

and because a single CKEditor 4 button could map to multiple CKEditor 5 toolbar items.