[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.

tools/sysid/src/main/native/cpp/analysis/FilteringUtils.cpp

@@ -179,7 +179,7 @@ sysid::TrimStepVoltageData(std::vector<PreparedData>* data,
     // Find maximum speed reached
     const auto maxSpeed =
         GetMaxSpeed(*data, [](auto&& pt) { return pt.velocity; });
-    // Find place where 90% of maximum speed exceeded
+    // Find place where 90% of maximum velocity exceeded
     auto endIt = std::find_if(
         data->begin(), data->end(), [&](const PreparedData& entry) {
           return std::abs(entry.velocity) > maxSpeed * 0.9;