dkg's 2021 OpenPGP transition

As 2021 begins, I'm changing to a new OpenPGP certificate.

I did a similar transition two years ago, and a fair amount has changed since then.

You might know my old OpenPGP certificate as:

pub   ed25519 2019-01-19 [C] [expires: 2021-01-18]
      C4BC2DDB38CCE96485EBE9C2F20691179038E5C6
uid          Daniel Kahn …