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>
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