[commands] Mark CommandPtr class as [[nodiscard]] (#7803)

This has the same effect but makes it so any user code returning CommandPtr can't discard a returned command.

Signed-off-by: Eric Ward <ezeward4@gmail.com>

wpilibcExamples/src/main/cpp/examples/RapidReactCommandBot/include/subsystems/Pneumatics.h

@@ -17,7 +17,6 @@ class Pneumatics : frc2::SubsystemBase {
  public:
   Pneumatics();
   /** Returns a command that disables the compressor indefinitely. */
-  [[nodiscard]]
   frc2::CommandPtr DisableCompressorCommand();
 
   /**