libstdc++: Add monadic operations to std::expected for C++23 (P2505R5)

libstdc++: Add monadic operations to std::expected for C++23 (P2505R5)

This was approved for C++23 last month in Kona.

libstdc++-v3/ChangeLog:

	* include/std/expected (expected): Add monadic operations.
	(expected<void, E>): Likewise.
	* include/std/version (__cpp_lib_expected): Bump value.
	* testsuite/20_util/expected/synopsis.cc: Adjust expected macro
	value.
	* testsuite/20_util/expected/version.cc: Likewise.
	* testsuite/20_util/expected/illformed_neg.cc: Prune additional
	errors from ill-formed monadic operations.
	* testsuite/20_util/expected/observers.cc: Check error_or.
	* testsuite/20_util/expected/monadic.cc: New test.