[boost] [contract] new maintenance release for Boost 1.72

Hello Release Managers & all, I made a maintenance release of Boost.Contract. All tests pass, docs are updated, etc. The code is up-to-date in boost/contract/master. Can you please pull changes from boost/contract/master into the upcoming Boost 1.72 release? These are the changes: * Using std::uncaught_exceptions on C++17 compilers onward (instead of the now obsolete std::uncaught_exception). * Fixed a few warnings. * Removed linking to Boost.System (Boost.System is now a header-only library). * Added this library to Boost's Continuous Integration (CI). * Fixed documentation typos. Thank you in advance. --Lorenzo