libstdc++: do not advertise full P2562R1 support

libstdc++: do not advertise full P2562R1 support

P2562R1 ("constexpr Stable Sorting") adds constexpr to stable_sort,
stable_partition and inplace_merge. However only the first is already
implemented in libstdc++, so we shouldn't bump the feature-testing
macro to the bumped C++26 value. This commit sets it to one less
than the final value.

Amends r15-7708-gff43f9853d3b10.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (constexpr_algorithms): Change
	the value of the feature-testing macro.
	* include/bits/version.h: Regenerate.
	* testsuite/25_algorithms/cpp_lib_constexpr.cc: Amend the
	check of the feature-testing macro.