pkgbase: Add instructions for upgrading past 78847e1

The pkg solver gets confused by the file moves and refuses to upgrade
without manually upgrading FreeBSD-utilities and FreeBSD-utilities-dev
first.

PR: 254173, 273859
MFC after: 1 week
Reviewed by: Mina Galić <freebsd@igalic.co>
Differential Revision: https://reviews.freebsd.org/D41892