[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/AddressableLED/robot.py

@@ -32,8 +32,8 @@ class MyRobot(wpilib.TimedRobot):
         self.kLedSpacing = 1 / 120.0
 
         # Create a new pattern that scrolls the rainbow pattern across the LED strip, moving at a
-        # speed of 1 meter per second.
-        self.scrollingRainbow = self.rainbow.scrollAtAbsoluteSpeed(
+        # velocity of 1 meter per second.
+        self.scrollingRainbow = self.rainbow.scrollAtAbsoluteVelocity(
             1,
             self.kLedSpacing,
         )