freebsd-update: Allow upgrade from pre-release builds

Update r343122 to include -ALPHA, -BETA and -RC releases as
upgrade-able via freebsd-update.

PR: 234771
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by: delphij, des
Reviewed by: delphij
MFC with: r343122
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18881