libstdc++: Use __glibcxx_ranges_as_const to guard P2278R4 changes

libstdc++: Use __glibcxx_ranges_as_const to guard P2278R4 changes

The P2278R4 additions for C++23 are currently guarded by a check for
__cplusplus > 202002L but can use __glibcxx_ranges_as_const instead.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (const_iterator_t): Change
	preprocessor condition to use __glibcxx_ranges_as_const.
	(const_sentinel_t, range_const_reference_t): Likewise.
	(__access::__possibly_const_range, cbegin, cend, crbegin)
	(crend, cdata): Likewise.
	* include/bits/stl_iterator.h (iter_const_reference_t)
	(basic_const_iterator, const_iterator, const_sentinel)
	(make_const_iterator): Likewise.