libpqxx: 6.2.2 -> 6.2.3

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/libpqxx/versions.

These checks were done:

- built on NixOS
- ran ‘/nix/store/6mshwwzcsvpxxwn4hw9msm5ffwyy5wv3-libpqxx-6.2.3/bin/pqxx-config -h’ got 0 exit code
- ran ‘/nix/store/6mshwwzcsvpxxwn4hw9msm5ffwyy5wv3-libpqxx-6.2.3/bin/pqxx-config --help’ got 0 exit code
- ran ‘/nix/store/6mshwwzcsvpxxwn4hw9msm5ffwyy5wv3-libpqxx-6.2.3/bin/pqxx-config help’ got 0 exit code
- found 6.2.3 with grep in /nix/store/6mshwwzcsvpxxwn4hw9msm5ffwyy5wv3-libpqxx-6.2.3
- directory tree listing: https://gist.github.com/509fd3c73bc4d5401153a2eb6e21a804