Upgrade from 12.2 to 13.1

[Cath O'Deray]

IIRC Ed Maste mentioned a couple of things missing from this part of the FreeBSD Handbook.

It took me a couple of days to remember where this was mentioned. Found: <https://reviews.freebsd.org/D35108#795740>

… at least the FreeBSD handbook documentation of freebsd-update appears to be missing a step or two.

The FreeBSD Handbook … no hint of the fetch and install (not an upgrade) that may be appropriate prior to freebsd-update upgrade -r 13.1-RELEASE.

FreeBSD bugs:

• 264113 – FreeBSD 13.1-RELEASE Installation Instructions: update
• 264142 – Handbook section 24.2.3; Add "freebsd-update fetch install" step before version upgrade – Glen Barber (Lead, Primary Release Engineering Team; FreeBSD Documentation Engineering Team) discussed with Pau Amma.