staff/support: recommend ssh-keygen over sshign

Use the standard tool shipped with OpenSSH instead of asking users
to download a third-party tool.

staff/support.md

@@ -23,7 +23,11 @@ Preferred factors include:
 
 - DKIM signatures on the email
 - PGP signatures on the email
-- SSH key challenges (see [sshign](https://git.sr.ht/~minus/sshign))
+- SSH key challenges
+  - To sign: `ssh-keygen -Y sign -n file -f ~/.ssh/<key> message.txt`
+  - To verify: check that `ssh-keygen -l -f <key>` and
+    `ssh-keygen -Y check-novalidate -n file -s message.txt.sig <message.txt`
+    match
 - DNS challenges on their mail server address or the domain in their profile
   (e.g. "please add this random string to a DNS TXT record to verify your
   identity")