syscons: refer to it as the legacy console

vt(4) is the default console, and although there is no firm deprecation
plan for syscons(4) yet it it is not actively maintained and is not
compatible with contemporary systems (i.e., those booting via UEFI).

Reviewed by: manu
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 2afef29b2c0b59182c6c1e133733943eafab714d)