Our support policy says that we can update to any Rust released at
least 6 months ago; 1.56 came out on 21 October 2021.

This doesn't yet change any code: it just increases the version
we say we need in our README, and the version we test against in
CI.

Our main justification for this change is to be able to upgrade to
newer versions of our dependencies, including `async_executors` >=
0.5, `aes` >= 0.8, and `cipher` >= 0.4.