Move to C++20: issues to be solved

[omoerbeek]

I did a few experiments to compile our codebase with C++20. These are my findings

1. el-7, debian-buster: forget it, but these are dead within a month anyway.
2. Minimum version of boost is 1.54 (due to a c++20 incompatibility). This has consequences for el-8
3. debian-bullseye needs a more modern compiler (likely clang, as the newest gcc there is version 10, which is too old). It also has a too old boost. This one needs a bit more effort.
4. ubuntu-focal: both boost and gcc are too old, but it wil be dead in 6 months.
5. The other platforms seem fine. Though only compile tested, no test of executables.

BRanch has the experiment for el-9, which works now, but the spec file is missing conditions, so it breaks other el builds.

To conclusion at this point is: there are a few nice features in c++20, but there is no pressing need to switch to it. If it does not take too much effort to do debian-bullseye, it is worth the trouble to switch after el-7, debian-buster and ubuntu-focal are dead.