Change mir_navigation to use move_base_flex by default

[Heenne]

The MiR100 and MiR200 launch files have the move_base_flex selector set to move_base by default. Change this to move_base_flex

[Heenne]

In https://github.com/match-ROS/match_mobile_robotics/blob/noetic/mir/mir_launch_sim/launch/mir_100.launch and https://github.com/match-ROS/match_mobile_robotics/blob/noetic/mir/mir_launch_sim/launch/mir_600.launch move_base is selected by default. Change this to move_base_flex and maybe also set it as default in the lower launch files