Remove references to portsnap for newer FreeBSD versions.

Provide instructions around the existing references for portsnap about how to
fetch ports for FreeBSD versions after 12.1 or 11.4, respectively.

While here:

• replace a stray public address with a RFC1918 private address
• move a plural outside of the <filename> tags
• update FreeBSD versions in poudriere examples

Submitted by: pauamma_gundo.com
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D25800