cutting-edge: Include boot environment use in a major upgrade with pkgbase

Explain the major version upgrade procedure with ZFS boot environments
as an extra safety net in case something goes wrong. Add a workaround
for a problem that may remove pkg itself in some edge cases. Remove
older, unsupported 14.2-RELEASE from the instructions.

Reviewed by: bcr, Graham Perrin <grahamperrin@gmail.com>
Differential Revision: https://reviews.freebsd.org/D53534