chore: php8.1 __serialize & __unserialize

to remove the php8.1 warning

Author: ice6

lib/Mongo/MongoId.php

@@ -142,6 +142,22 @@ public function unserialize($serialized)
         $this->createObjectID($serialized);
     }
 
+    /**
+     * @return string
+     */
+    public function __serialize()
+    {
+        return (string) $this->objectID;
+    }
+
+    /**
+     * @param string $serialized
+     */
+    public function __unserialize($serialized)
+    {
+        $this->createObjectID($serialized);
+    }
+
     /**
      * Gets the incremented value to create this id
      * @link http://php.net/manual/en/mongoid.getinc.php