Support the `rust-version` field in `Cargo.toml` to specify the MSRV

[jonasbb]

The first crates are starting to make use of the rust-version field in Cargo.toml, which gets stabilized with the 1.56 version of rust, see rust-lang/rust#65262 and rust-lang/cargo#9732.

So far it does not seem like Clippy is able to understand this field. It only supports specifying it in it's own config file and unstably as an inner attribute (Readme).

@rustbot label +C-enhancement

[flip1995]

@rustbot claim