Rethink deprecation of the <deal.II/base/std_cxx17/*> header files.

[bangerth]

@masterleinad A question:

When you deprecated the headers in deal.II/base/std_cxx17 in #15563, the idea was presumably that these headers were no longer necessary for users because we now require C++17 and so they shouldn't #include them any more. But that jumps the gun (I think). For example, ASPECT can currently be compiled with deal.II 9.4, 9.5, and master. With 9.4 and 9.5, if I include <deal.II/base/std_cxx17/optional.h>, for example, I end up with symbols std_cxx17::optional that I can use in the code. But if I use master, then the #include ends up with an unconditional warning from the compiler. I could work around this by only #includeing this file if DEAL_II_HAVE_CXX17 is not set, and #include <optional> instead, but then the class I want is no longer available in namespace std_cxx17.

I think what we should have done is to do an "early deprecate" on the header file -- or perhaps not deprecate it at all right away, and instead leave that for after the next release. Thoughts?

[masterleinad]

I just followed what we did for the std_cxx14 headers but given that use DEAL_II_DEPRECATED_EARLY for other deprecations, we should probably guard the warning with DEAL_II_EARLY_DEPRECATIONS. #15736 does that.

[tamiko]

Easy fix. The deprecations will give a warning in two years and the header will be removed in three.