[FEA] Require C++20

[bdice]

Many RAPIDS libraries are moving to require C++20, now that we require a minimum of CUDA 12 (which supports C++20). I would like to evaluate moving RMM to require C++20 as well. ~This will target the 25.12 release, probably.~ Update: targeting this for a future release, depending on requirements gathered.

We already require C++20 for compiling benchmarks, and have some notes about where C++20 can be used to improve existing code:

• https://github.com/rapidsai/rmm/blob/163c84a5c5f11ef20b26c3740782189456167e5c/cpp/benchmarks/utilities/log_parser.hpp#L109

• https://github.com/rapidsai/rmm/pull/1573#discussion_r1627257620

[bdice]

I'd like to gather a list of projects using RMM that require C++17 support and cannot move to C++20.

Currently it sounds like ucxx might be on that list (cc: @pentschev). If anyone is aware of others, please list them here.

[bdice]

@trivialfis -- sounds like XGBoost may have a requirement for C++17. Please share any additional information you have.

[trivialfis]

Please go ahead, we will figure something out.