libpqxx: 6.2.3 -> 6.2.4 (#41547)

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
- /nix/store/d309iqn144gbq4g5dd2w2jnbqdi4ywrz-libpqxx-6.2.4/bin/pqxx-config passed the binary check.
- 1 of 1 passed binary check by having a zero exit code.
- 0 of 1 passed binary check by having the new version present in output.
- found 6.2.4 with grep in /nix/store/d309iqn144gbq4g5dd2w2jnbqdi4ywrz-libpqxx-6.2.4
- directory tree listing: https://gist.github.com/9ba0b82a0fae29ca1da4ffd51bf8c2e2
- du listing: https://gist.github.com/eb156369be72c9249ae90e6536eb3537