committers-guide: make it clear that Submitted by is deprecated

Add (deprecated) in the first column. When mentioned only in prose in
the second column it may be missed.

Noticed by: rmacklem
Reviewed by: jrtc27, lwhsu
Event: Kitchener-Waterloo Hackathon 2023
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40196