Depecrate "skip all"

The feature makes little sense, allowing skipping of "all supported traits", a group that cannot be expanded in a non-breaking change. This will still keep the possibility to skip all traits directly, just remove the "skip all" behavior. I.e. use ```rs #[derive_where(skip(Debug, EqHashOrd, Zeroize /*, Clone */))] ``` instead of ```rs #[derive_where(skip)] ```