Switch to C++20

[julianbrost]

(No particular plan yet, I'd just like to know what the CI thinks of that idea.)

[Al2Klimov]

Spoiler alert

• For generic types' methods, C++20 doesn't like e.g ~ObjectImpl<SyslogLogger>(): build/lib/base/sysloglogger-ti.hpp:52:9: error: template-id not allowed for destructor
• IIRC, older Boost versions are also affected by this kind of error
• RHEL 8 and SLES 15.7 use Boost 1.66, AL2 uses v1.69

So that won't work.