Require C++17

As discussed in https://mail.kde.org/pipermail/plasma-devel/2020-August/118263.html we agreed that requiring C++17 in Plasma is fine.
There's no code that actually uses C++17 features yet, but specifying the requirement anyway has advantages.

It documents the policy in a much more discoverable place than a wiki page or similar could do
Building in 17 mode allows the compiler to do some stuff implicitly, like guranteed copy elision