Fix ackermann demo

[christophfroehlich]

To fix #438 I changed the definition of the steering joints (and also used meaningful inertial parameters).

While reproducing #438 I found another issue with the example, the launch file crashed and didn't even start.

I also changed the deprecated Twist command to TwistStamped to avoid

[ruby $(which ign) gazebo-2] [WARN] [1735417503.762406833] [ackermann_steering_controller]: Use of Twist message without stamped is deprecated and it will be removed in ROS 2 J-Turtle version. Use '~/reference' topic with 'geometry_msgs::msg::TwistStamped' message type in the future.