gvinum: add deprecation notice

Vinum is a Logical Volume Manager that was introduced in FreeBSD 3.0,
and for FreeBSD 5 was ported to geom(4) as gvinum. gvinum has had no
specific development at least as far back as 2010, and has a number of
known bugs which are unlikely to be resolved.

Add a deprecation notice to raise awareness but state that vinum "may
not be" available in FreeBSD 14. Either it will be removed and the
notice will be updated to "is not" available, or someone will step up
to fix issues and maintain it and we will remove the notice.

Reviewed by: imp (earlier version)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29424