sidenav.html (sidebar at pages such as /where/): remove 13.0, add 14.0

D36520 – https://reviews.freebsd.org/D36520 – intended to, but did
not, include an essential update to the sidebar.

Fixing my omission, attention to the section of sidenav.html –

.sidenavType "download"

– that currently forms the traditional sidebar at pages such as:

• Download FreeBSD (also known as Get FreeBSD)
• FreeBSD Snapshot Releases (a.k.a. Snapshot Releases)
• Release Engineering Information
• Release Information.

PR: 266167
Reviewed by: gjb (mentor)
Approved by: gjb
Fixes: b1fcdc95159

home, sidebars, /releases/: updates, corrections

Pull request: https://github.com/grahamperrin/freebsd-doc/pull/3
Differential revision: https://reviews.freebsd.org/D36676