`std::jthread` availability

HI,
Latest version 0.8.0 requires c++20, it should be directly integrated into Package.swift using something like this :
```
cxxSettings: [
            .unsafeFlags(["-std=c++20"], .when(platforms: [.iOS]))
        ]
```

In fact you do have it. Wonder why it is "ignored". I get errors about missing c++20 features