Declarative launch files

[mhubii]

Refer to ros2_control: ros-controls/ros2_control_demos#982 (comment)

In the ROSCon livestream, it was explained how Python launch files (designed to be backend), are incorrectly be used in the frontend.

A quick fix would be moving towards declerative launch files.

Demands changes for the lbr_bringup package. Mixins might be kept for now.

[mhubii]

For future reference:

• The mixins will need to be removed from the lbr_bringup package.
• The MoveIt launch files will need to be updated.
• The launch files will likely be replaced with yaml-style (now default recommendation for frontend usage)