Use nested namespace

It's a mechanical change to modernize the code base

src/engine/plugins/nearest.cpp

@@ -7,11 +7,7 @@
 #include <boost/assert.hpp>
 #include <boost/numeric/conversion/cast.hpp>
 
-namespace osrm
-{
-namespace engine
-{
-namespace plugins
+namespace osrm::engine::plugins
 {
 
 NearestPlugin::NearestPlugin(const int max_results_) : max_results{max_results_} {}
@@ -57,6 +53,4 @@ Status NearestPlugin::HandleRequest(const RoutingAlgorithmsInterface &algorithms
 
     return Status::Ok;
 }
-} // namespace plugins
-} // namespace engine
 } // namespace osrm