Automate MSRV change in the crate doc

I noted some months ago that the environment variable CARGO_PKG_RUST_VERSION was publicized in 1.63.0 and that if we do not go back before 1.63 then we can automate the MSRV there.
Remain "ci.yml" but I don't see a way to automate it, even with `cargo metadata --no-deps --format-version=1 | jq -r '.packages[0].version'`.

Author: Philippe-Cholet

src/lib.rs

@@ -49,7 +49,9 @@
 //!
 //! ## Rust Version
 //!
-//! This version of itertools requires Rust 1.63.0 or later.
+//! This version of itertools requires Rust
+#![doc = env!("CARGO_PKG_RUST_VERSION")]
+//! or later.
 
 #[cfg(not(feature = "use_std"))]
 extern crate core as std;