[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.

wpilibc/src/main/native/cpp/hardware/expansionhub/ExpansionHubMotor.cpp

@@ -92,10 +92,10 @@ ExpansionHubMotor::~ExpansionHubMotor() noexcept {
   m_hub.UnreserveMotor(m_channel);
 }
 
-void ExpansionHubMotor::SetPercentagePower(double power) {
+void ExpansionHubMotor::SetDutyCycle(double dutyCycle) {
   SetEnabled(true);
   m_modePublisher.Set(kPercentageMode);
-  m_setpointPublisher.Set(power);
+  m_setpointPublisher.Set(dutyCycle);
 }
 
 void ExpansionHubMotor::SetVoltage(wpi::units::volt_t voltage) {