Switch to ed25519 as default key type

[dolmen]

ed25519 is the algorithm that GitHub recommends: https://docs.github.com/en/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent

[dolmen]

This has been released in v1.400.

[dolmen]

Some top open source contributors or cryptography experts use (ecdsa-sha2-nistp256) instead:

• Filippo Valsorda (@FiloSottile): public keys, history
• Russ Cox (@rsc): public keys, history
• Jon Geater (@JAG-UK): public keys

Relevant papers:

• https://essay.utwente.nl/75354/1/DNSSEC%20curves.pdf

[FiloSottile]

I use ecdsa-sha2-nistp256 because it's what's supported by my YubiKey which I use through yubikey-agent.

If I used a local key it would probably be an ed25519, although the difference is not likely to ever matter.

[JAG-UK]

I have p256 for very similar reasons to @FiloSottile - in other contexts (esp. web3 type engagements) I've been happily using 25519 for years. No issues with this change.