Update package version to 5.6.2 and remove unused prop

Author: kosuke-k

package.json

@@ -1,6 +1,6 @@
 {
   "name": "react-lightning-design-system",
-  "version": "5.6.1",
+  "version": "5.6.2",
   "description": "Salesforce Lightning Design System components built with React",
   "main": "lib/scripts/index.js",
   "module": "module/scripts/index.js",

src/scripts/DropdownMenu.tsx

@@ -92,7 +92,6 @@ export type DropdownMenuItemProps = {
   onClick?: (e: React.SyntheticEvent) => void;
   submenu?: ReactNode;
   submenuItems?: Array<{ key: string | number } & DropdownMenuItemProps>;
-  openSubmenuKey?: string | number;
   level?: number;
 } & Omit<AnchorHTMLAttributes<HTMLAnchorElement>, 'onClick'>;