pkg.conf: Use kmod repositories

The kmod repositories were added shortly after 14.2-RELEASE. Point at
them here so that users don't need to manually add the configuration
in order to get e.g. working graphics.

MFC after: 1 minute
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D50583

(cherry picked from commit 29f076ae0cb536d1e891d3375edf28f1bc82df79)