Deprecate IterativeRobot in favor of TimedRobot (#1341)

Since https://github.com/wpilibsuite/allwpilib/issues/786 has been
closed as not a legitimate concern, there is now no reason to use
IterativeRobot over TimedRobot. It's a drop-in replacement that's
strictly an improvement in terms of execution jitter.

To migrate, one simply has to replace the IterativeRobot subclass in
their robot code with TimedRobot.

wpilibc/src/main/native/include/frc/IterativeRobot.h

@@ -22,6 +22,9 @@ namespace frc {
  */
 class IterativeRobot : public IterativeRobotBase {
  public:
+  WPI_DEPRECATED(
+      "Use TimedRobot instead. It's a drop-in replacement that provides more "
+      "regular execution periods.")
   IterativeRobot();
   virtual ~IterativeRobot() = default;