restrict certificate type for builtin SSH server (#26789)

- While doing some sanity checks over OpenSSH's code for how they handle
certificates authentication. I stumbled on an condition that checks the
certificate type is really an user certificate on the server-side
authentication. This checks seems to be a formality and just for the
sake of good domain seperation, because an user and host certificate
don't differ in their generation, verification or flags that can be
included.
- Add this check to the builtin SSH server to stay close to the
unwritten SSH specification.
- This is an breaking change for setups where the builtin SSH server is
being used and for some reason host certificates were being used for
authentication.
- 
(cherry picked from commit de35b141b79a3d6efe2127ed2c73fd481515e481)

Refs: https://codeberg.org/forgejo/forgejo/pulls/1172

## ⚠️ BREAKING ⚠️

Like OpenSSH, the built-in SSH server will now only accept SSH user
certificates, not server certificates.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-authored-by: Giteabot <teabot@gitea.io>

Author: 3 people

modules/ssh/ssh.go

@@ -191,6 +191,12 @@ func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
 			return false
 		}
 
+		if cert.CertType != gossh.UserCert {
+			log.Warn("Certificate Rejected: Not a user certificate")
+			log.Warn("Failed authentication attempt from %s", ctx.RemoteAddr())
+			return false
+		}
+
 		// look for the exact principal
 	principalLoop:
 		for _, principal := range cert.ValidPrincipals {