added mixed return type

wordpress-objects/Clarkson_Object.php

@@ -651,8 +651,7 @@ public function has_term( $term ) {
 	 *
 	 * We can't just return $this->_post, because these values will only return raw unfiltered data.
 	 */
-	#[\ReturnTypeWillChange]
-	public function jsonSerialize() {
+	public function jsonSerialize(): mixed {
 		$data['id']        = $this->get_id();
 		$data['link']      = $this->get_permalink();
 		$data['slug']      = $this->get_post_name();