Get FreeBSD a.k.a. Download FreeBSD: remove 13.0

The presence of end-of-life (EOL) content is sometimes a source of
confusion, especially for newcomers to FreeBSD.
https://lists.freebsd.org/archives/freebsd-hackers/2022-September/001468.html
there's emphasis on timely updates, when ends of life occur.

Now: remove the 13.0 section i.e.
https://www.freebsd.org/where/#download-rel130; then we can have a
new diff for, and more leisurely review of, D36523
https://reviews.freebsd.org/D36523.

Reported by: Mason Loring Bliss and others
Reviewed by: gjb (mentor)
Approved by: gjb (mentor)
Pull Request: https://github.com/grahamperrin/freebsd-doc/pull/2