Swap `reqwest` for `ureq`, at least in the io crate

[DanGould]

We originally used ureq so that we could build on MSRV 1.57.0. However, now that we target 1.63.0, reqwest is a viable option again and reqwest is far more common as a dependency in our downstream targets. We should target it if only to reduce a second dependency for downstream projects who would like to use the io crate.