Suppress deprecation notice for php 8.1+ (#242)

Since php 8.1 internal functions are using proper return types. This results in deprecation notices for some methods. To supress this for php8.1 the ReturnTypeWillChange attribute is introduced. For more info check out https://stitcher.io/blog/new-in-php-81#interal-method-return-types-rfc

Author: samybasset

src/Optimizely/OptimizelyUserContext.php

@@ -147,6 +147,7 @@ public function getOptimizely()
         return $this->optimizelyClient;
     }
 
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return [