[Development] Switching Qt default builds to C++20 where supported

[Thiago Macieira]

On Monday 28 October 2024 08:10:42 Pacific Daylight Time Volker Hilsheimer via 
Development wrote:
> As long as we can guarantee that the binaries we distribute with this change
> are source- and binary-compatible with what is today released without this
> change, and as long as the other conclusions from the previous discussions
> hold (e.g. C++20 is not required, neither to build nor to use Qt), I’m not
> going to stop anyone from spending time on this.
> 
> I don’t know if and how we can guarantee that across all platforms. And, as
> Vladimir indicated: CI bandwidth is limited, and we will be prioritising
> that bandwidth for those configurations that we are going to support in the
> field, e.g. if in doubt and necessary, explicitly selecting C++17. Which
> might then just as well stay the default.

My changes will likely switch most of the CI builds to C++20, as described in 
the earlier email. How many we maintain at C++17 is TBD and can be as low as 
zero.

Header-checking for C++17 will be implemented.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCAI Platform & System Engineering
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