libstdc++: Remove dg-options "-std=gnu++20" from 21_strings 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/21_strings/basic_string/cons/char/105995.cc: Remove
	dg-options -std=gnu++20.
	* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/erasure.cc: Likewise.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/literals/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/swap/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operators/char/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operators/wchar_t/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/cons/char/range.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/copy/char/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/copy/wchar_t/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/char/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/wchar_t/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/requirements/constexpr_iter.cc:
	Likewise.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
	Likewise.