try reading rust-version from Cargo.toml
Cargo.toml can contain a field `rust-version`, that acts like a MSRV of
clippy.toml file: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field
This will try to read that field and use it, if the clippy.toml config
has no `msrv` entry
changelog: respect `rust-version` from `Cargo.toml`
closes#8746closes#7765