Swap all `idna::domain_to_ascii` to `idna::domain_to_ascii_cow`

[`idna::domain_to_ascii` docs](https://docs.rs/idna/latest/idna/fn.domain_to_ascii.html) say: > This function exists for backward-compatibility. Consider using [`domain_to_ascii_cow`](https://docs.rs/idna/latest/idna/fn.domain_to_ascii_cow.html) instead. In the process, an `idna::AsciiDenyList` argument will be added. `EMPTY` would retain the previous behavior; `URL` may be preferred in some cases.