[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/DriveDistanceOffboard/subsystems/drivesubsystem.py

@@ -63,7 +63,7 @@ class DriveSubsystem(commands2.Subsystem):
 
         self.profile = wpimath.TrapezoidProfile(
             wpimath.TrapezoidProfile.Constraints(
-                constants.DriveConstants.kMaxSpeed,
+                constants.DriveConstants.kMaxVelocity,
                 constants.DriveConstants.kMaxAcceleration,
             )
         )