add return type to JsonSerializable::jsonSerialize()

add return type to JsonSerializable::jsonSerialize() implementations to suppress deprecation notice on PHP 8.1

Deprecated: Return type of JSONAPI\OAS\OAuthFlow::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in .../vendor/bednic/json-api/src/OAS/OAuthFlow.php on line 74