libstdc++: Correct value of __cpp_lib_constexpr_exceptions [PR117785]

libstdc++: Correct value of __cpp_lib_constexpr_exceptions [PR117785]

Only P3068R6 (Allowing exception throwing in constant-evaluation) is
implemented in the library so far, so the value of the
constexpr_exceptions feature test macro should be 202411L. Once we
support the library changes in P3378R2 (constexpr exception types) then
we can set the value to 202502L again.

libstdc++-v3/ChangeLog:

	PR libstdc++/117785
	* include/bits/version.def (constexpr_exceptions): Define
	correct value.
	* include/bits/version.h: Regenerate.
	* libsupc++/exception: Check correct value.
	* testsuite/18_support/exception/version.cc: New test.