libstdc++: Implement P2836R1 changes to const_iterator

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (const_iterator): Define conversion
	operators as per P2836R1.
	* include/bits/version.def (ranges_as_const): Update value.
	* include/bits/version.h: Regenerate.
	* testsuite/24_iterators/const_iterator/1.cc (test04): New test.
	* testsuite/std/ranges/adaptors/as_const/1.cc: Adjust expected
	value of __cpp_lib_ranges_as_const.
	* testsuite/std/ranges/version_c++23.cc: Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>