pump up to v2.3.9 and do release

[cvvergara]

[PR #306] and requires a release.

• pump up to 2.3.9
• check if any other issue has a fast fix for the release

[phpwutz]

can this be released sometime soon?
I'm currently trying to build osm2pgrouting from the AUR and it just won't build due to this problem

osm2pgrouting/src/osm2pgrouting-2.3.8/src/osm_elements/osm_tag.cpp:34:18: error: ‘transform’ is not a member of ‘std’
   34 |             std::transform(value.begin(), value.end(), value.begin(), [](char ch) {
      |                  ^~~~~~~~~

[h-vetinari]

Any updates on a release? :)