converters/osm2pgrouting: fix build with recent libpqxx

Recent versions of libpqxx require C++20 to be used.
Adjust the requested C++ standard in CMakeLists.txt.

Approved by: portmgr (build fix blanket)
MFH: 2026Q2

(cherry picked from commit 7bfaddbcafafd8fc1d5649d75f64af46b4a095e3)