Add lint to detect when the `rust-version` field is not defined

[sorairolake]

What it does

Checks to see if the rust-version field is defined in Cargo.toml.

Advantage

The MSRV-aware resolver takes this field into account when picking dependencies if it is defined. If this field is defined, the toil is reduced for maintainers to support older toolchains by not needing to manually select older versions for each dependency.

Drawbacks

The MSRV-aware resolver has been stabilized since Rust 1.84.0.

Example

[package]
# ...

Could be written as:

[package]
# ...
rust-version = "1.56"

[Manishearth]

Closing, see discussion in #14071