libstdc++: Remove dg-options "-std=gnu++23" from std::expected tests

The testsuite will automatically select C++23 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++26
as well.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/expected/assign.cc: Remove dg-options.
	* testsuite/20_util/expected/bad.cc: Likewise.
	* testsuite/20_util/expected/cons.cc: Likewise.
	* testsuite/20_util/expected/equality.cc: Likewise.
	* testsuite/20_util/expected/illformed_neg.cc: Likewise.
	* testsuite/20_util/expected/lwg3877.cc: Likewise.
	* testsuite/20_util/expected/lwg3938.cc: Likewise.
	* testsuite/20_util/expected/monadic.cc: Likewise.
	* testsuite/20_util/expected/observers.cc: Likewise.
	* testsuite/20_util/expected/requirements.cc: Likewise.
	* testsuite/20_util/expected/swap.cc: Likewise.
	* testsuite/20_util/expected/synopsis.cc: Likewise.
	* testsuite/20_util/expected/unexpected.cc: Likewise.
	* testsuite/20_util/expected/version.cc: Likewise.