bsdconfig, bsdinstall: Refresh mirrors list

The mirrors list is in sync with the Handbook / Mirrors section [1],
which was refreshed a few months ago. Mirrors removed were not
responding or had duplicated addresses (aliases) with another mirror.

1 - https://docs.freebsd.org/en/books/handbook/mirrors/

Reviewed by: philip (clusteradm)
Approved by: philip
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38014