FAQ change

[Robert J. Hansen]

Due to Yahoo! Groups closing, the PGPNET mailing list has moved to
groups.io; the FAQ has been updated with the change.  Nobody objected to
this, so it seems like a safe change.

Due to a lack of any consensus for how the existing text should change,
I've made no other edits at this time.  This is a problem for us,
because we need to figure out whether we wish to continue advocating SKS
and how we ought adjust the RSA-2K language in the FAQ in light of the
fact we currently default to -3K and will soon move to ECC.