Require C++20
This was proposed a while ago (https://mail.kde.org/pipermail/plasma-devel/2021-August/120545.html) with no blockers raised.
Note that this doesn't mean we can rely on any C++20 feature, we still need to decide that on a case-by-case basis, based on what the relevant compilers (i.e. gcc and clang) actually implement.