C++ version bump for Plasma

[Sune Vuorela]

On 2024-12-16, Xaver Hugl <xaver.hugl at kde.org> wrote:
> We're planning to use some C++ 23 features in KWin, namely for Plasma
> 6.3 std::expected. This is supported in GCC 12 and Clang 16, so if

Debian stable has gcc 12 and clang 16, so at least from that viewpoint
it should be okay.

Are std::expected considered stable in those versions, not only
existing?

/Sune