libstdc++: Remove dg-options "-std=gnu++20" from 27_io tests

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499.cc:
	Remove dg-options -std=gnu++2a.
	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc:
	Likewise.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499.cc:
	Likewise.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc:
	Likewise.
	* testsuite/27_io/basic_istringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/cons/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_istringstream/str/char/2.cc: Likewise.
	* testsuite/27_io/basic_istringstream/str/wchar_t/2.cc:
	Likewise.
	* testsuite/27_io/basic_istringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_istringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/emit/1.cc: Likewise.
	* testsuite/27_io/basic_ostream/inserters_character/char/deleted.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_character/wchar_t/deleted.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/cons/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/str/char/3.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/str/wchar_t/3.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/cons/char/2.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/str/char/4.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/str/wchar_t/4.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/view/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/cons/char/97719.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/cons/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/str/char/5.cc: Likewise.
	* testsuite/27_io/basic_stringstream/str/wchar_t/5.cc.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/view/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringstream/view/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/basic_syncbuf/1.cc: Likewise.
	* testsuite/27_io/basic_syncbuf/2.cc: Likewise.
	* testsuite/27_io/basic_syncbuf/basic_ops/1.cc: Likewise.
	* testsuite/27_io/basic_syncbuf/requirements/types.cc: Likewise.
	* testsuite/27_io/basic_syncbuf/sync_ops/1.cc: Likewise.
	* testsuite/27_io/basic_syncstream/1.cc: Likewise.
	* testsuite/27_io/basic_syncstream/2.cc: Likewise.
	* testsuite/27_io/basic_syncstream/basic_ops/1.cc: Likewise.
	* testsuite/27_io/basic_syncstream/requirements/types.cc:
	Likewise.
	* testsuite/27_io/filesystem/iterators/106201.cc: Likewise.
	* testsuite/27_io/filesystem/iterators/lwg3719.cc: Likewise.
	* testsuite/27_io/filesystem/path/factory/u8path-depr.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/nonmember/cmp_c++20.cc:
	Likewise.