[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.

wpiunits/src/main/java/org/wpilib/units/ImmutableMeasure.java

@@ -7,7 +7,7 @@ package org.wpilib.units;
 import org.wpilib.units.measure.Dimensionless;
 
 /**
- * A measure holds the magnitude and unit of some dimension, such as distance, time, or speed. An
+ * A measure holds the magnitude and unit of some dimension, such as distance, time, or velocity. An
  * immutable measure is <i>immutable</i> and <i>type safe</i>, making it easy to use in concurrent
  * situations and gives compile-time safety. Two measures with the same <i>unit</i> and
  * <i>magnitude</i> are effectively equivalent objects.

wpiunits/src/main/java/org/wpilib/units/Measure.java

@@ -33,8 +33,8 @@ import org.wpilib.units.measure.Velocity;
 import org.wpilib.units.measure.Voltage;
 
 /**
- * A measure holds the magnitude and unit of some dimension, such as distance, time, or speed. Two
- * measures with the same <i>unit</i> and <i>magnitude</i> are effectively equivalent objects.
+ * A measure holds the magnitude and unit of some dimension, such as distance, time, or velocity.
+ * Two measures with the same <i>unit</i> and <i>magnitude</i> are effectively equivalent objects.
  *
  * @param <U> the unit type of the measure
  */

wpiunits/src/main/java/org/wpilib/units/Units.java

@@ -164,7 +164,7 @@ public final class Units {
 
   /**
    * A unit of angular velocity equivalent to spinning at a rate of one {@link #Rotations Rotation}
-   * per {@link #Minute}. Motor spec sheets often list maximum speeds in terms of RPM.
+   * per {@link #Minute}. Motor spec sheets often list maximum velocities in terms of RPM.
    */
   public static final AngularVelocityUnit RotationsPerMinute = Rotations.per(Minute);