doc: remove expiration from PGP key in security.html

The key had expired earlier this year. Simply resigned it with no
expiration, so it maintains the same fingerprint.

Removed the encouragement to use PGP above the fold. We trust the
security of our mail system, so it's really only there for people that
want it.

Also removed the individual keys, as they were never used, and both Adam
and I have access to the security@golang.org key anyway.

Change-Id: Icc5ad6dfb4f0b52128a59a080b7f270b20d3c520
Reviewed-on: https://go-review.googlesource.com/c/go/+/211177


Reviewed-by: Katie Hockman <katie@golang.org>