Make mlkem768x25519-sha256 the preferred KEX by default

It is reportedly faster than sntrup761x25519-sha512, and on Java 24+ can
use the JDK's built-in ML-KEM.

Author: tomaswolf

sshd-core/src/main/java/org/apache/sshd/common/BaseBuilder.java

@@ -87,9 +87,9 @@ public class BaseBuilder<T extends AbstractFactoryManager, S extends BaseBuilder
      */
     public static final List<BuiltinDHFactories> DEFAULT_KEX_PREFERENCE = Collections.unmodifiableList(
             Arrays.asList(
+                    BuiltinDHFactories.mlkem768x25519,
                     BuiltinDHFactories.sntrup761x25519,
                     BuiltinDHFactories.sntrup761x25519_openssh,
-                    BuiltinDHFactories.mlkem768x25519,
                     BuiltinDHFactories.mlkem1024nistp384,
                     BuiltinDHFactories.mlkem768nistp256,
                     BuiltinDHFactories.curve25519,