bug #3536 Add ReturnTypeWillChange to JsonSerializable implementation (derrabus)

This PR was merged into the 2.x branch.

Discussion
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Add ReturnTypeWillChange to JsonSerializable implementation

On PHP 8.1, implementations of `JsonSerializable` should declare a return type.

In order to avoid a deprecation warning and maintain backwards compatibility, I've added the `ReturnTypeWillChange` attribute to declare that our implementation will add the necessary return type in a future release.

Commits
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507f78d9 Add ReturnTypeWillChange to JsonSerializable implementation

src/Markup.php

@@ -40,6 +40,7 @@ class Markup implements \Countable, \JsonSerializable
         return mb_strlen($this->content, $this->charset);
     }
 
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->content;