Update documentation references, and double-check implementation against RFC9987 #116

In May of this year, the SSH agent protocol graduated to become RFC 9987 🎉

Currently, all the documentation in this library (other than the bits I've updated in #114) still reference draft-miller-ssh-agent-14, which has some (albeit minor) differences compared to the final RFC.

Creating this as a catch-all issue to cover:

• Updating documentation to reference RFC 9987, and
• Identifying any areas where we might break the spec - I haven't identified any yet, but probably a good time to do so!