Use PostgreSQL v18 client libraries

[citrus-it]

As part of preparing for updating to HeliosV3, update the PostgreSQL client libraries used by
the pq-sys crate and nexus from version 13 to version 18. This is a version that is available
in both Heliosv2 and Heliosv3, easing migration later.

This will likely require developers to update build machines

% pfexec pkg install library/postgresql-18
% pkg set-mediator -V 18 postgresql

Or re-running tools/install_builder_prerequisites.sh should do the same thing.