[Qt-creator] Requiring C++20 for Creator 13

[apoenitz]

Hi all.

I'd like to find out whether anyone would be ill-affected in any way
if we'd require C++20 to compile current master branch, i.e. what
will become Qt Creator 13 next spring.

To be clear: That's only about building Qt Creator itself, this does
not affect code build with Qt Creator at all.

So if you are building Qt Creator yourself (or have a friend who does)
and you use (or rather "need to use") a compiler that doesn't support
C++20 out-of-the box, feel free to drop a note here or contact me
in private mail.

Andre'