Add deprecation notice to vpo.4

This driver has seen no real changes for almost 20 years. It's for
hardware that's 25 years old. It has no reports of active use, nor
has it been seen in the NYCBug dmesg database at all. Schedule
its removal for 13.0.

Reviewed by: rgrimes@ (earlier version)
Relnote: Yes
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D23403