Nest all namespaces

The compiler supports this now.  We can type less going forward.
No functional changes.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I29d6fa4f9aacc0e381f1a7637294db0392466995
diff --git a/y2023/control_loops/drivetrain/drivetrain_base.cc b/y2023/control_loops/drivetrain/drivetrain_base.cc
index a11a896..63e7904 100644
--- a/y2023/control_loops/drivetrain/drivetrain_base.cc
+++ b/y2023/control_loops/drivetrain/drivetrain_base.cc
@@ -15,9 +15,7 @@
 
 namespace chrono = ::std::chrono;
 
-namespace y2023 {
-namespace control_loops {
-namespace drivetrain {
+namespace y2023::control_loops::drivetrain {
 
 using ::frc971::constants::ShifterHallEffect;
 
@@ -85,6 +83,4 @@
   return kDrivetrainConfig;
 };
 
-}  // namespace drivetrain
-}  // namespace control_loops
-}  // namespace y2023
+}  // namespace y2023::control_loops::drivetrain