Generate a new core secretary PGP key. It is signed with the old core
secretary key and by matthew@, the outgoing core secretary.

% gpg --list-key core-secretary@freebsd.org
pub rsa2048/0x0CB403E4E95B96EC 2018-06-30 [SC] [expires: 2020-06-29]

Key fingerprint = 9F02 836F 50D3 AD5A B75A  C588 0CB4 03E4 E95B 96EC

uid [ultimate] FreeBSD Core Team Secretary <core-secretary@freebsd.org>
sub rsa2048/0x133C3338A5B95A60 2018-06-30 [E] [expires: 2020-06-29]

Approved by: matthew (outgoing core secretary)
Differential Revision: https://reviews.freebsd.org/D16077