* configure.ac: Activate C++17, and replace --enable-c++17 by
--enable-c++20.
* NEWS: Mention the news.
* .gitlab-ci.yml: Use C++20 for the former C++17 builds.
* HACKING, README, doc/org/compile.org, doc/org/concepts.org,
doc/org/index.org, doc/org/install.org, doc/org/tut.org,
doc/org/upgrade2.org, spot/misc/escape.hh: Adjust mentions
of C++14.