libstdc++: Remove std::allocator::is_always_equal typedef for C++26

This was removed by P2868R3, voted into the C++26 draft at the November
2023 meeting in Kona. We've had a deprecated warning in place for three
years.

libstdc++-v3/ChangeLog:

	* include/bits/allocator.h (allocator::is_always_equal): Do not
	define for C++26.
	(allocator<void>::is_always_equal): Likewise.
	* testsuite/20_util/allocator/requirements/typedefs.cc: Check
	that is_always_equal is not present in C++26.
	* testsuite/20_util/allocator/void.cc: Do not require
	is_always_equal for C++26.
	* testsuite/23_containers/vector/bool/cons/constexpr.cc: Add
	missing override of base's is_always_equal.
	* testsuite/23_containers/vector/cons/constexpr.cc: Likewise.