Wdeprecated-declarations from GCC v10.1.0

Including `json.hpp v3.7.3` and compiling with `g++ v10.1.0` using `-std=c++20`, triggers the following warning:

```
json.hpp:12595:28: warning: ‘template<class _Tp> struct std::is_pod’ is deprecated: 
use is_standard_layout && is_trivial instead [-Wdeprecated-declarations]
12595 |         static_assert(std::is_pod<CharType>::value, "CharType must be POD");
```