archivers/py-borgbackup: clean up 1.4.0 upgrade fallout

• Mark broken on FreeBSD 13 due to https://github.com/borgbackup/borg/issues/8269 which upstream maintainer intends to fix for borgbackup 1.4.1 (milestone). Reported by "Support SimpleRezo" in direct e-mail message.

• Add UPDATING entry. The broken MOVED entry has been removed by sunpoet@ already,

Reported by: Juraj Lutter <otis@>, freebsd@rail.eu.org
PR: 280424

• Move py-pkgconfig to BUILD_DEPENDS to fix build failures in poudriere or on bare metal if py-pkgconfig isn't installed.

Reported by: Juraj Lutter <otis@>, Hyogeol Lee <hyogeollee@gmail.com> (patch)
PR: 280413