LO 24.2 C++20 baseline

[David Gerard]

> 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".

Unfortunately, this breaks the current Ubuntu LTS, 22.04 - LibreOffice
can't be compiled on it any more:

configure: error: GCC 11.4.0 is too old, must be at least GCC 12
Error running configure at /home/fun/libreoffice/autogen.sh line 321.
make: *** [Makefile:56: /home/fun/libreoffice/config_host.mk] Error 1

$ gcc --version
gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0

For gcc 12, as i understand it I'd have to try to bodge in the debs
from 22.10 (the current experimental version) or compile it from
source.

The next Ubuntu LTS is 24.04.


- d.