Deprecate references of portsnap.

Portsnap references post 12.1-RELEASE are added to note the new way to fetch
ports. Details can be found here:

https://lists.freebsd.org/pipermail/freebsd-ports/2020-August/119098.html

Submitted by: pauamma_gundo.com
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D25808