LO 24.2 C++20 baseline

[Stephan Bergmann]

I just submitted
<https://git.libreoffice.org/core/+/1eef07805021b7ca26a1a8894809b6d995747ba1%5E%21>
"Bump baseline to C++20" to master.  I will let it sit like that for a
few more days, to see if anything breaks (Coverity? Linux distros'
needs? etc.), before cleaning up now-unnecessary `#if __cplusplus <
202002L` etc., and before potentially unleashing the rewriting Clang
plugin to mass-introduce uses of u""_ustr that I talked about
yesterday at <https://events.documentfoundation.org/libreoffice-conference-2023/talk/U9XTSL/>
"If I Had C++20".