Prevent deprecation warning when updating to PHP 8.1 while preserving PHP 8.0 compatibility.

This will allow the lib user to make sure they don't have deprecation warnings on their way to 8.1 update

Author: Dean151

README.md

@@ -1,6 +1,6 @@
 # Pushok
 
-[![PHP >= 7.2](https://img.shields.io/badge/php-%3E%3D%208.0-8892BF.svg?style=flat-square)](https://php.net/)
+[![PHP >= 8.0](https://img.shields.io/badge/php-%3E%3D%208.0-8892BF.svg?style=flat-square)](https://php.net/)
 [![Build Status][ico-travis]][link-travis]
 [![Latest Version on Packagist][ico-version]][link-packagist]
 [![Total Downloads][ico-downloads]][link-downloads]

src/Payload.php

@@ -458,6 +458,7 @@ public function toJson(): string
      * @return array
      * @link   http://php.net/manual/en/jsonserializable.jsonserialize.php
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         $payload = self::getDefaultPayloadStructure();

src/Payload/Alert.php

@@ -325,6 +325,7 @@ public function toJson(): string
      * @return array
      * @link   http://php.net/manual/en/jsonserializable.jsonserialize.php
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         $alert = [];

src/Payload/Sound.php

@@ -139,6 +139,7 @@ public function toJson(): string
      * @return array
      * @link   http://php.net/manual/en/jsonserializable.jsonserialize.php
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         $sound = [];