[commands] Mark command group lifecycle methods as final (#4385)

This prevents accidental footguns due to overriding of command group lifecycle methods.

wpilibNewCommands/src/main/native/include/frc2/command/ParallelCommandGroup.h

@@ -74,13 +74,13 @@ class ParallelCommandGroup
     AddCommands(std::move(foo));
   }
 
-  void Initialize() override;
+  void Initialize() final;
 
-  void Execute() override;
+  void Execute() final;
 
-  void End(bool interrupted) override;
+  void End(bool interrupted) final;
 
-  bool IsFinished() override;
+  bool IsFinished() final;
 
   bool RunsWhenDisabled() const override;