Fix jsonSerialize on PHP 8.1+ (#71)

* Fix jsonSerialize on PHP 8.1+

```
During inheritance of JsonSerializable: Uncaught ErrorException: Return type of FlixTech\SchemaRegistryApi\Schema\AvroReference::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

* Use ReturnTypeWillChange to be PHP 7 compatible

src/Schema/AvroReference.php

@@ -13,7 +13,6 @@ final class AvroReference implements \JsonSerializable
      */
     private $name;
 
-
     /**
      * @var string
      */
@@ -37,6 +36,7 @@ public function __construct(AvroName $name, string $subject, $version)
         $this->version = $version;
     }
 
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return [