Deprecate C++11 and C++14 support in libcu++

[bernhardmgruber]

We should add a deprecation warning to libcu++ that suport for the C++11 and C++14 dialects is deprecated. This warning should be suppressable by a macro the same way it can be supressed for CUB and Thrust.

Background

Thrust and CUB deprecated C++11 and C++14 in the past and issue deprecation warnings when compiling in those standards. Both libraries will move to C++17 at the next major release. Experimental libraries like cudax already require C++17 today. While libcu++ started in C++14 initially, many C++11 backports were added over time and libcu++ is mostly written in C++11/14 at the moment. As last project in CCCL, it's time to begin moving libcu++ to C++17 as well.

Motivation

Maintaining support for C++11 and C++14 incurs significant costs in resources for building and testing, developer time, compilation time for our users, cuts us off from modern features to provide better APIs, and thus ultimately diminishes the value we can deliver to users. Moving the entire CCCL to C++17 will:

• Reducing CI cost by cutting the build matrix in half (from C++11/14/17/20 to 17/20)
• Shift developer effort from backports and workarounds to providing more features and improve performance
• Provide better code by reducing implementation complexity, have less bugs, faster compiles, and better+safer APIs

End of C++11/14 support

The deprecation warning should be added soon to get the word out. The eventual removal of C++11/14 support is being discussed internally, and more information will be communicated when a conclusion has been reached.

But I need C++11/14!

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