libstdc++: Add std::span::at for C++26 (P2821R5)

Also define the new feature test macros from P2833R2, indicating that
std::span and std::expected are supported for freestanding mode.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (freestanding_expected): New macro.
	(span): Add C++26 value.
	* include/bits/version.h: Regenerate.
	* include/std/expected (__glibcxx_want_freestanding_expected):
	Define.
	* include/std/span (span::at): New member function.
	* testsuite/20_util/expected/version.cc: Add checks for
	__cpp_lib_freestanding_expected.
	* testsuite/23_containers/span/2.cc: Moved to...
	* testsuite/23_containers/span/version.cc: ...here. Add checks
	for __cpp_lib_span in <span> as well as in <version>.
	* testsuite/23_containers/span/1.cc: Removed.
	* testsuite/23_containers/span/at.cc: New test.