ASV: suppress return type deprecation warning

Forward compatibility for PHP 8.1:
This annotation suppresses the
 "Deprecated: Return type of mle86\Value\AbstractSerializableValue::
 jsonSerialize() should either be compatible with JsonSerializable::
 jsonSerialize(): mixed" warning
(without having to add an explicit "mixed" return type
which would not work with PHP7).

src/AbstractSerializableValue.php

@@ -34,6 +34,7 @@ public function __toString(): string
      *
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->value();