Try to use C++20

[herzbube]

Xcode 13 introduced C++20 support. Try to use it in the project. Search for TODOs in the code that indicate where this may be beneficial. Have a look at libsgfc++ which, I think, also wants to use a higher C++ version for some filesystem stuff.

Possibly this requires going to a newer IPHONEOS_DEPLOYMENT_TARGET, ditching some old devices.