Style(9) tweak

From: Warner Losh <imp_at_bsdimp.com>
Date: Thu, 03 Feb 2022 05:45:22 UTC
Greetings,

Please review https://reviews.freebsd.org/D34152.

I've strengthened the admonition against $FreeBSD$. After this change,
style(9) recommends that you only include $FreeBSD$ in new code you are
sure will be merged to stable/12. If you later decide you need to merge
code to stable/12, you can always add $FreeBSD$ just before the merge.
Also, since stable/11 is no longer supported, I've removed references to it
as well.

Thanks!

Warner