libstdc++: Do not declare namespace ranges in <numeric> unconditionally

Move namespace ranges inside the feature test macro guard, because
'ranges' is not a reserved name before C++20.

libstdc++-v3/ChangeLog:

	* include/std/numeric (ranges): Only declare namespace for C++23
	and later.
	(ranges::iota_result): Fix indentation.
	* testsuite/17_intro/names.cc: Check ranges is not used as an
	identifier before C++20.