`zvariant_derive` is unmaintained

The `zvariant_derive` crate is no longer maintained as a standalone crate.

Its derive macros are provided by [`zbus`](https://crates.io/crates/zbus) starting with version `6.0.0`.

Users should migrate to [`zbus`](https://crates.io/crates/zbus), with its default features disabled if they only need the D-Bus wire format.