libstdc++: Implement std::to_string in terms of std::format (P2587R3)

This change for C++26 affects std::to_string for floating-point
arguments, so that they should be formatted using std::format("{}", v)
instead of using sprintf. The modified specification in the standard
also affects integral arguments, but there's no observable difference
for them, and we already use std::to_chars for them anyway.

To avoid <string> depending on all of <format>, this change actually
just uses std::to_chars directly instead of using std::format. This is
equivalent, because the format spec "{}" doesn't use any of the other
features of std::format.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (to_string(floating-point-type)):
	Implement using std::to_chars for C++26.
	* include/bits/version.def (__cpp_lib_to_string): Define.
	* include/bits/version.h: Regenerate.
	* testsuite/21_strings/basic_string/numeric_conversions/char/dr1261.cc:
	Adjust expected result in C++26 mode.
	* testsuite/21_strings/basic_string/numeric_conversions/char/to_string.cc:
	Likewise.
	* testsuite/21_strings/basic_string/numeric_conversions/wchar_t/dr1261.cc:
	Likewise.
	* testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring.cc:
	Likewise.
	* testsuite/21_strings/basic_string/numeric_conversions/char/to_string_float.cc:
	New test.
	* testsuite/21_strings/basic_string/numeric_conversions/wchar_t/to_wstring_float.cc:
	New test.
	* testsuite/21_strings/basic_string/numeric_conversions/version.cc:
	New test.