periodic: by default, skip 221.backup-gpart in jails

It can still be enabled as usual in /etc/periodic.conf

PR: 256253
Reported by: delphij
Submitted by: Miroslav Lachman <000.fbsd@quip.cz>

(cherry picked from commit fcf2227a557552e45646bbcf2422a98baab5c8a8)