Replace declaration lambdas.

[kamiradi]

          There is a more modern way to do this using lambdas, which avoids needing these weird placeholders

    robot_description_subscriber_ = node_->create_subscription<std_msgs::msg::String>(
        "robot_description", rclcpp::SystemDefaultsQoS(), [this](const std_msgs::msg::String::SharedPtr msg){
            if (!description_set_)
    {
      robot_description_ = msg->data;
      description_set_ = true;
    }};

_Originally posted by @sjahr in https://github.com/moveit/moveit_drake/pull/8#discussion_r1663990536_

[kamiradi]

Resolved in #18