By using conditional compilation, we now support running the test suite
with Rust versions 1.83 to 1.86 again. This allows us to lower the
`rust-version` specified in the root Cargo.toml (because it was
controlling the toolchain used in CI) to 1.83, resolving #230.
This solution keeps tests operational on our MSRV while also lowering
it. It would have been unsatisfying to have an MSRV which could not
compile the tests.
`rustversion` was selected as the dependency to control the conditional
compilation since it was already a build dependency needed by
`vergen-gitcl`, so no new dependencies were added.
