Address "ssh-rsa" deprecation in OpenSSH

[stevenjohnstone]

Ran into this in #347.

OpenSSH plans to deprecate "ssh-rsa". Some distros are already moving in this direction. In particular, when STIGs are applied in RHEL, "ssh-rsa" is not allowed. Trouble is that golang.org/x/crypto SSH doesn't negotiate

There's work on going for this upstream e.g. https://go-review.googlesource.com/c/crypto/+/360195. With the latest (golang.org/x/crypto v0.0.0-20211215153901-e495a2d5b3d3), negotiating ssh with "ssh-rsa" deprecated does not work with RSA keys.

[kke]

Still waiting for golang/crypto#197

[james-nesbitt]

no longer relevant. launchpad doesn't manage keys, and it can handle ed-25519 now.