14.4/errata: add issue - freebsd-update hanging when pkg is not bootstrapped

Add an erratum describing a problem where freebsd-update(8) commands such as
fetch or install may hang when pkg(8) is not bootstrapped.

The erratum also documents a workaround: bootstrap pkg(8) before running
freebsd-update(8).

Reviewed by: ziaee, grahamperrin
Approved by: ziaee, re (implicit)
Differential Revision: https://reviews.freebsd.org/D55754
PR: 293640