upgt: Deprecate ancient 802.11b/g driver

Recently-reported bugs in this driver should be fixed, but also the
hardware it supports is obsolete so mark it for deprecation.

PR: 284876
Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49045

(cherry picked from commit 7f8a5c5a1585fd9b56adfd8de4f9194ba5cc0785)