Use nested namespace

It's a mechanical change to modernize the code base

src/engine/plugins/match.cpp

@@ -16,11 +16,7 @@
 #include <set>
 #include <vector>
 
-namespace osrm
-{
-namespace engine
-{
-namespace plugins
+namespace osrm::engine::plugins
 {
 
 // Filters PhantomNodes to obtain a set of viable candidates
@@ -314,6 +310,4 @@ Status MatchPlugin::HandleRequest(const RoutingAlgorithmsInterface &algorithms,
 
     return Status::Ok;
 }
-} // namespace plugins
-} // namespace engine
 } // namespace osrm