RPC: add deprecation warning for v1-v4 usage

With FastAPI starting to come closer to a close, we've got to advertise this deprecation so that users have some time to adjust before making the changes. We have not specified a specific time here, but we'd like this message to reach users of the RPC API for at least a month before any modifications are made to the interface.

Signed-off-by: Kevin Morris kevr@0cost.org

This MR relates to #100 (closed)