use rpc from neutron-lib
We recently removed neutron.common.rpc  as the rpc module lives in
neutron-lib now. This project was missed because I didn't find
neutron.common.rpc as an import.
This patch switches rpc uses over to neutron-lib.
This patch also wraps in a needed update to lower-constraints.txt
(cannot merge separately, because both changes are needed to
unbreak the CI)
Co-Authored-By: Akihiro Motoki <email@example.com>