Set twist_mux/use_stamped to default false, to match twist_marker

[TWALL9]

Something I noticed when updating the workspace I've been using for my hobby projects. The use_stamped parameter in twist_marker is defaulting to false, but the same parameter in twist_mux must be explicitly set or the node will throw an exception and die.

I'm making an assumption that when use_stamped was added to the twist_mux node, it was intentionally added without a default value to force developers to evaluate their usage of the node. I actually agree with that direction, if that was the intention, but at the same time it's pretty annoying that the two nodes in this package have different behaviours where one is optional and the other is not.

This PR changes /twist_mux/use_stamped to default to false when the node is created, and declares it as a parameter for twist_mux instead of just blindly looking in the global namespace.

There is also the matter that Steve M brought up regarding the default value of this parameter with Nav2. . I don't address that issue here, it's orthoganal to the merits of having a parameter act uniformly across the package.

[bmagyar]

The marker should be updated on this case. The better half of the ROS universe has finally moved on to use stamped twist messages, let's not regress on that.