P2937R0 Freestanding Library: Remove `strtok`

WG21-P2937R0 Freestanding Library: Remove `strtok`

**INCREASED** feature-test macro (expected):
```cpp
#define __cpp_lib_freestanding_cstring 202311L
```

We're certain that the impact of this paper for our hosted implementation is to increase the value of the feature-test macro that #3837 added, so we can accept a PR for this before we open up the floodgates to arbitrary C++26 features.