[wpimath] Replace Speeds with Velocities (#8479)

I left "free speed" alone since that's the technical term for it. In
general, velocity is a vector quantity, and speed is a magnitude (i.e.,
a strictly positive value).

This PR also replaces the speed verbiage in MotorController with duty
cycle.

Fixes #8423.

robotpyExamples/ElevatorExponentialProfile/examplesmartmotorcontroller.py

@@ -74,7 +74,7 @@ class ExampleSmartMotorController:
         """Resets the encoder to zero distance."""
         pass
 
-    def set(self, speed: float) -> None:
+    def set(self, velocity: float) -> None:
         pass
 
     def get(self) -> float: