Dependency's `rust-version` is incompatible with the current toolchain

### What it does This is an existing diagnostic that we want to turn into a lint. This can only happen once the linting system is stable (#12235) to avoid regressions. This is part of #9930. ### Advantage Provide more actionable error messages Being able to control this existing diagnostic unblocks a `package.rust-version = "toolchain"` (or whatever we call it) ### Drawbacks If the rust-version higher than it absolutely has to, then this could be limiting (which is why we want to turn this into a lint) ### Example N/A