Don't force public variables to use Hungarian notation (#8774)

People generally have expressed a dislike for the Hungarian notation
used in member variables, especially in examples/templates, and our
styleguide shouldn't be forced on downstream consumers, so this removes
all Hungarian notation from the examples/templates.

There are _some_ benefits to Hungarian for private member variables
(like knowing what's a member vs. local in a PR review) so we'll keep
private member variables the same for now, but public variables should
no longer use Hungarian notation, since it looks much worse. A new PMD
XPath rule has been added to accomplish this goal. Some other
non-compliant variables were fixed for the new rule.

wpilibj/src/main/java/org/wpilib/drive/DifferentialDrive.java

@@ -68,7 +68,6 @@ public class DifferentialDrive extends RobotDriveBase implements Sendable, AutoC
    *
    * <p>Uses normalized voltage [-1.0..1.0].
    */
-  @SuppressWarnings("MemberName")
   public static class WheelVelocities {
     /** Left wheel velocity. */
     public double left;

wpilibj/src/main/java/org/wpilib/drive/MecanumDrive.java

@@ -70,7 +70,6 @@ public class MecanumDrive extends RobotDriveBase implements Sendable, AutoClosea
    *
    * <p>Uses normalized voltage [-1.0..1.0].
    */
-  @SuppressWarnings("MemberName")
   public static class WheelVelocities {
     /** Front-left wheel velocity. */
     public double frontLeft;