Use nested namespace

It's a mechanical change to modernize the code base

src/server/service/tile_service.cpp

@@ -8,11 +8,7 @@
 
 #include <boost/format.hpp>
 
-namespace osrm
-{
-namespace server
-{
-namespace service
+namespace osrm::server::service
 {
 
 engine::Status TileService::RunQuery(std::size_t prefix_length,
@@ -47,6 +43,4 @@ engine::Status TileService::RunQuery(std::size_t prefix_length,
     result = std::string();
     return BaseService::routing_machine.Tile(*parameters, result);
 }
-} // namespace service
-} // namespace server
 } // namespace osrm