libstdc++: Only define basic_string::contains for C++23

libstdc++: Only define basic_string::contains for C++23

The new contains member of the COW string is defined for non-strict
gnu++20 mode as well as for C++23 modes. I think that was left in the
committed patch unintentionally. It is inconsistent with the SSO string,
and doesn't actually compile because it uses the
basic_string_view::contains member which only defined for C++23.

This makes it only defined for C++23.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/cow_string.h (basic_string::contains): Do not
	define for -std=gnu++20.