doc: update minimum supported FreeBSD to 12.2

FreeBSD 11 goes out of support on 2021-09-30.
FreeBSD 12.1 is not supported since 2021-01-31.

PR-URL: https://github.com/nodejs/node/pull/40179
Refs: https://en.wikipedia.org/wiki/FreeBSD_version_history#Version_history


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>