[Bug libstdc++/81469] std::uncaught_exception should be marked as deprecated for C++1z

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81469

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We'll probably also want to adjust any tests for uncaught_exception so they
don't give a deprecated warning when run with RUNTESTFLAGS=unix/-std=c++17