[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.

wpilibcExamples/src/main/cpp/examples/Xrptimed/include/Robot.hpp

@@ -27,6 +27,6 @@ class Robot : public wpi::TimedRobot {
   wpi::Timer m_timer;
 
   wpi::DifferentialDrive m_drive{
-      [&](double output) { m_leftMotor.Set(output); },
-      [&](double output) { m_rightMotor.Set(output); }};
+      [&](double output) { m_leftMotor.SetDutyCycle(output); },
+      [&](double output) { m_rightMotor.SetDutyCycle(output); }};
 };