deprecate sane-frontends

This code is not maintained anymore by upstream (last release: 14 years
ago as per https://gitlab.com/sane-project/frontends/-/releases), has no
maintainer here (anymore), no consumers and is prone to breakage (last
fixed in ports r517748). As per discussion with tobik@, deprecate this;
the expiration date gives ample warning to interested users but avoids
carrying this port into the next quarterly.