libstdc++: Simplify C++20 poison pill overloads (P2602R2)

libstdc++: Simplify C++20 poison pill overloads (P2602R2)

This implements the C++23 change "Poison Pills are Too Toxic". This
makes sense to do unconditionally for C++20, as the corner cases that it
fixes are considered to be defects in the C++20 design (e.g. LWG3480 was
needed to fix directory iterators because of these pills being too
toxic).

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (__imove::iter_move): Define
	poison pill as deleted for consistency.
	(__access::begin): Replace with a single declaration.
	* include/bits/ranges_base.h (__access::end, __access::rbegin)
	(__access::rend, __access::size): Likewise.
	* include/bits/version.def (ranges): Update value for C++23.
	* include/bits/version.h: Regenerate.
	* libsupc++/compare (__compare): Add missing poison pill
	overloads.
	* testsuite/std/ranges/version_c++23.cc: Adjust expected value
	of __cpp_lib_ranges.
	* testsuite/std/ranges/access/p2602.cc: New test.