Make libsane-hpaio suggest hplip instead of recommend

The preferred approach documented in https://wiki.debian.org/SaneOverNetwork with modern driverless scanners is that libsane, potentially with one or two driver packages, is able to reach most network and USB scanners without additional vendor tooling such as hplip. Indeed the documentation there suggests avoiding hplip even when only libsane-hpaio is needed. Downgrade the Recommends: hplip to a Suggests: to make this the default behaviour when adding libsane-hpaio to your system.