Have a CI job that makes sure we build with our oldest dependencies

We should have a job that tries to install the oldest version of each of our dependencies that is still compatible with our Cargo.toml files, and then makes sure that we still build and that our tests still pass. This will ensure that we aren't listing any versions older than we require.

cargo update --minimal-versions might be a part of this solution, but that flag seems to act recursively over our dependencies' dependencies, which exposes not only our bugs, but theirs too.