Migrate `mrobosub_planning/stop.py`

[imzaynb]

@RCoder01 brings up a good point that this file would be run sporadically to zero out the motor outputs in case the last state/command had left the motors in a non-zero output state.

Can this be migrated to ros2 semantics?

[imzaynb]

You'll likely have to go back a commit on devel since we are finally merging ros2 to devel.