ssh: export supported algorithms

Fixes golang/go#61537

Change-Id: If3478121e3ae445391e3faeceeb889d75e9e3214
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/531935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

ssh/server_test.go

@@ -212,7 +212,7 @@ func TestNewServerConnValidationErrors(t *testing.T) {
 
 	serverConf = &ServerConfig{
 		Config: Config{
-			KeyExchanges: []string{kexAlgoDHGEXSHA256},
+			KeyExchanges: []string{KeyExchangeDHGEXSHA256},
 		},
 	}
 	c = &markerConn{}