PHP 8.1: fix deprecation notice

As of PHP 8.1, PHP will start throwing deprecation warnings along the lines of:
```
Deprecated:  Return type of [CLASS]::[METHOD]() should either be compatible with [PHP NATIVE INTERFACE]::[METHOD](): [TYPE], or the #[ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

These type of deprecation notices relate to the [Return types for internal methods RFC](https://wiki.php.net/rfc/internal_method_return_types) in PHP 8.1.

Basically, as of PHP 8.1, these methods in classes which implement PHP native interfaces are expected to have a return type declared.
The return type can be the same as used in PHP itself or a more specific type. This complies with the Liskov principle of covariance, which allows the return type of a child overloaded method to be more specific than that of the parent.

The return type for `JsonSerializable::jsonSerialize()` is supposed to be `mixed`, but as that is a PHP 8.0 type, it cannot yet be added.
Ref: https://www.php.net/manual/en/class.jsonserializable.php

With that in mind, I've added the annotation instead.

Note: While attributes are a PHP 8.0 feature only, due to the syntax choice for `#[]`, they will ignored in PHP < 8 and can be safely added.

src/Token/AccessTokenInterface.php

@@ -15,6 +15,7 @@
 namespace League\OAuth2\Client\Token;
 
 use JsonSerializable;
+use ReturnTypeWillChange;
 use RuntimeException;
 
 interface AccessTokenInterface extends JsonSerializable
@@ -68,5 +69,6 @@ public function __toString();
      *
      * @return array
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize();
 }