Use nested namespace

It's a mechanical change to modernize the code base

src/extractor/compressed_edge_container.cpp

@@ -11,9 +11,7 @@
 
 #include <iostream>
 
-namespace osrm
-{
-namespace extractor
+namespace osrm::extractor
 {
 
 CompressedEdgeContainer::CompressedEdgeContainer()
@@ -399,5 +397,4 @@ std::unique_ptr<SegmentDataContainer> CompressedEdgeContainer::ToSegmentData()
 
     return std::move(segment_data);
 }
-} // namespace extractor
 } // namespace osrm