Specify rust-version in Cargo.toml matching wayland-rs

This prevents Clippy from showing lints to use features added in later
Rust versions.

Sctk seems to build with this Rust version, so it seems reasonable to
set that target for now.

Cargo.toml

@@ -6,6 +6,7 @@ documentation = "https://smithay.github.io/client-toolkit"
 repository = "https://github.com/smithay/client-toolkit"
 license = "MIT"
 edition = "2021"
+rust-version = "1.65"
 categories = ["gui"]
 keywords = ["wayland", "client"]
 description = "Toolkit for making client wayland applications."