Consider deprecating the crate

[pentamassiv]

Relm is depending on the gtk crates. However they are unmaintained. New versions have been published that warn users about the deprecation, the Github repo has been archived and a Rust advisory was posted (gtk-rs/gtk4-rs#1935). It might make sense to also deprecate the relm crate, since the foundation it is built on is now unmaintained. I also did the same for my crate (https://crates.io/crates/gtk-layer-shell), that depends on gtk. It's best to close a project when it fulfilled it's purpose.