[12.x] Prevent relation autoload context from being serialized (#55582)

Author: litvinchuk

src/Illuminate/Database/Eloquent/Model.php

@@ -2502,6 +2502,7 @@ public function __sleep()
         $this->classCastCache = [];
         $this->attributeCastCache = [];
         $this->relationAutoloadCallback = null;
+        $this->relationAutoloadContext = null;
 
         return array_keys(get_object_vars($this));
     }