Alpaka: Considerations for C++17-only release

[BenjaminW3]

CUDA 11 will add support for C++17. This would allow us to require C++17 as minimum version for alpaka which would allow us to simplify some internals.

As an intermediate step we should create a release that still supports c++14 but already adds support for CUDA 11 and c++17.

In a follow up release we could drop support for the following dependencies because they miss full c++17 support:

• CUDA 9.0, 9.1, 9.2, 10.0, 10.1, 10.2
• clang 4
• gcc 5 and 6

[psychocoderHPC]

Note: we need to stay with CUDA 10.1 requirements as long as the most used HPC systems using CUDA 10.1.
A stable release with C++14 will help but also increase the required work because we need to create back-ports and work on multible alpaka versions at the same time.

[bernhardmgruber]

As discussed in a VC: I think compiler/toolchain updates are governed by what kinds of (HPC-) systems we want to support. So if all of these systems have upgraded to a new compiler/toolchain, we can upgrade as well. @sbastrakov pointed me to a list of systems where picongpu has been run, so we have a (partially outdated) sample of these supported systems.

I think we should have a table of supported systems and their compiler/toolchain versions and then regularly (like every month) check if those have changed. This will allow us to know then the time for an update has come.

[ax3l]

Fantastic ideas, maybe we could do a simple wiki page in the Alpaka repo?