gpg-agent on Ubuntu 18.04 does not support rsa-sha2-512 and rsa-sha2-256 signature algorithms for SSH

[Stan Hu]

On Ubuntu 18.04 with a Yubikey RSA key, we noticed SSH logins were failing when we switched our SSH server out from OpenSSH to a custom Golang SSH server.

We noticed that gpg-agent v2.2.6 fixes this problem via https://github.com/gpg/gnupg/commit/80b775bdbb852aa4a80292c9357e5b1876110c00. We highly recommend Ubuntu 18.04 either backport this patch or upgrade to v2.2.6.

As described in https://github.com/golang/go/issues/53391, a SSH client initiates a session via a SSH_MSG_USERAUTH_REQUEST message:

```
     byte SSH_MSG_USERAUTH_REQUEST
     string user name
     string service name
     string "publickey"
     boolean TRUE
     string "rsa-sha2-512" <--- 1 - Public key algorithm name (algo) (https://datatracker.ietf.org/doc/html/rfc4252#page-8)
     string public key blob:
         string "ssh-rsa" <--- 2 - Public key type (pubKey.Type())
         mpint e
         mpint n
     string signature:
         string "rsa-sha2-512" <--- 3 - Signature format (sig.Format)
         string rsa_signature_blob
```

If a SSH server mandates that 1 and 3 match, the login will fail. OpenSSH is a bit more relaxed here in that as long as 3 is either `ssh-rsa`, `rsa-sha2-256`, or `rsa-sha2-512`, the signature verification is still allowed to proceed.

This problem occurs because while OpenSSH v7.6 supports these new algorithms, gpg-agent doesn't know about them, so it just passes along `ssh-rsa` in 3.