Merge branch '3.x' into 4.x

* 3.x:
  re-add mixed return type

src/Markup.php

@@ -42,6 +42,9 @@ class Markup implements \Countable, \JsonSerializable, \Stringable
         return mb_strlen($this->content, $this->charset);
     }
 
+    /**
+     * @return mixed
+     */
     public function jsonSerialize(): mixed
     {
         return $this->content;

src/Util/TemplateDirIterator.php

@@ -16,11 +16,17 @@ namespace Twig\Util;
  */
 class TemplateDirIterator extends \IteratorIterator
 {
+    /**
+     * @return string
+     */
     public function current(): mixed
     {
         return file_get_contents(parent::current());
     }
 
+    /**
+     * @return string
+     */
     public function key(): mixed
     {
         return (string) parent::key();