Switch to aws-lc-rs feature (default) of rustls
In our use of rustls we are currently linking against the ring crate, using the ring feature.
Ring is now marked as unmaintained.
Although dragging in an AWS maintained OpenSSL fork, we should switch to the aws-lc-rs feature (the default) of rustls. Relatedly, in !172 (merged) we disable the cargo-deny error about ring, because we also rely on rustls + ring via nethsm-sdk-rs->ureq.