RFC: C++ 20 minimum standard version

[lightsighter]

It's been almost three years since we made the jump to C++17 being the minimum required version for Legion. Considering the C++ release cadence is every three years, it seems pertinent that we should consider whether it is time to upgrade to making C++20 be the minimum requirement for Legion programs this year. That doesn't mean we have to do it, but we'll at least start the discussion for people that have opinions. C++20 seems to have pretty good support now among all the major compilers:

https://en.cppreference.com/w/cpp/compiler_support/20.html

I can think of three immediate features that I would start to use if we make the jump:

https://en.cppreference.com/w/cpp/container/span.html
https://en.cppreference.com/w/cpp/atomic/atomic_ref.html
https://en.cppreference.com/w/cpp/memory/make_obj_using_allocator.html

[lightsighter]

Adding to my list, I would also really like to use some of this functionality to greatly simplify a significant number of the changes required to support arenas:

https://en.cppreference.com/w/cpp/ranges.html

[lightsighter]

Another good one:

https://en.cppreference.com/w/cpp/utility/source_location.html

[opensdh]

We do not currently require C++20 for our users, but we're planning to do so very soon (both because of the features you're mentioning and because our users, who independently want them, have already been migrating). So no objections here.