POCO C++ Libraries Release 1.15 is Available

This is a major feature release with C++20 as the default standard, comprehensive
MongoDB replica set support, a new reactor-based HTTP server implementation, ULID
support, C++17 modernization of core classes, and significant third-party library
upgrades. For the details, please see the CHANGELOG.

Thanks to Matej Kenda for doing the majority of work on this release.