Update minimum supported gcc to gcc-9

### 🚀 The feature, motivation and pitch

As this is the first compiler that has full support for C++17 features, for example https://en.cppreference.com/w/cpp/header/filesystem or [`std::exclusive_scan`](https://en.cppreference.com/w/cpp/algorithm/exclusive_scan) (for more info, see  https://godbolt.org/z/q19MPs1rP )

### Alternatives

Do not use C++17

cc @seemethere