Eventually, upgrade to aes ≥0.8 and cipher≥0.4

Eventually we should upgrade to the latest version of the aes and cipher crates.

We shouldn't do so right now, since they list an MSRV of 1.56, which we don't yet allow.

When we do this, we'll need to be aware that these versions have significant API changes: I haven't been able to figure out how to make an AES-ctr cipher, for example.

edit: Maybe it's ctr::Ctr128BE<aes::128>?

Maybe by the time we're ready to upgrade, there will be some example code. 😅