P2836R1 `basic_const_iterator` Should Follow Its Underlying Type's Convertibility

WG21-P2836R1 `basic_const_iterator` Should Follow Its Underlying Type's Convertibility

**INCREASED** feature-test macro (expected):
```cpp
#define __cpp_lib_ranges_as_const 202311L
```

This was formally accepted as a Defect Report, retroactively amending C++23.
