[wpimath] Move PIDController from frc2 to frc namespace (#5640)

The old PIDController class in the frc namespace was removed for the
2023 season.

wpimath/src/test/native/cpp/controller/PIDInputOutputTest.cpp

@@ -8,9 +8,9 @@
 
 class PIDInputOutputTest : public testing::Test {
  protected:
-  frc2::PIDController* controller;
+  frc::PIDController* controller;
 
-  void SetUp() override { controller = new frc2::PIDController{0, 0, 0}; }
+  void SetUp() override { controller = new frc::PIDController{0, 0, 0}; }
 
   void TearDown() override { delete controller; }
 };