minor #4764 re-add mixed return type (xabbuh)

This PR was merged into the 3.x branch.

Discussion
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re-add mixed return type

re-doing #4582, see https://github.com/symfony/symfony/actions/runs/21779345389/job/62840897500#step:8:3635

Commits
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3a903664a8 re-add mixed return type

.php-cs-fixer.dist.php

@@ -18,6 +18,7 @@ return (new Config())
         'single_line_throw' => false,
         'ordered_imports' => true,
         'phpdoc_types_order' => ['null_adjustment' => 'always_last', 'sort_algorithm' => 'none'],
+        'no_superfluous_phpdoc_tags' => ['allow_mixed' => true],
     ])
     ->setRiskyAllowed(true)
     ->setParallelConfig(ParallelConfigFactory::detect())

src/Markup.php

@@ -46,6 +46,9 @@ class Markup implements \Countable, \JsonSerializable, \Stringable
         return mb_strlen($this->content, $this->charset);
     }
 
+    /**
+     * @return mixed
+     */
     #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {