micro-ros baudrate should be 921600 everywhere

There is a mismatch between baud rate in linorobot2 and linorobot2_hardware on the micro-ros baudrate. It is 921600 most places, but a few places are 115200, notably for pico. It needs to be 921600 to enable a 50Hz control loop rate. Baudrate should be the same in all configs unless there's some reason not to. 

This issue relates to:
https://github.com/linorobot/linorobot2/issues/174