MFC freebsd-update: Clarify unsupported release upgrade error message

MFC r343122:

freebsd-update: Clarify unsupported release upgrade error message

Notify users that upgrading from -CURRENT or -STABLE is unsupported by
freebsd-update.

Also ensure --currently-running provides a correctly formatted release
(as done by -r).

Reported by: yuri
Reviewed by: bcran

MFC r343268:

frebsd-update: fix --currently-running after r343122

Reported by: Brandon Schneider

MFC r343271:

freebsd-update: Allow upgrade from pre-release builds

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

Reported by: delphij, des
Reviewed by: delphij

MFC r343410:

freebsd-update: fix style from r343271 change

PR: 234771
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Sponsored by: The FreeBSD Foundation