Use nested namespace

It's a mechanical change to modernize the code base

src/engine/routing_algorithms/shortest_path.cpp

@@ -2,11 +2,7 @@
 #include "engine/routing_algorithms/routing_base_mld.hpp"
 #include "engine/routing_algorithms/shortest_path_impl.hpp"
 
-namespace osrm
-{
-namespace engine
-{
-namespace routing_algorithms
+namespace osrm::engine::routing_algorithms
 {
 
 template InternalRouteResult
@@ -21,6 +17,4 @@ shortestPathSearch(SearchEngineData<mld::Algorithm> &engine_working_data,
                    const std::vector<PhantomNodeCandidates> &waypoint_candidates,
                    const boost::optional<bool> continue_straight_at_waypoint);
 
-} // namespace routing_algorithms
-} // namespace engine
 } // namespace osrm