nixos/openssh: allow post-quantum KexAlgorithms introduced in v9.9 (#378875)

`mlkem768x25519-sha256` and the vendor-neutral `sntrup761x25519-sha512`
name are new in OpenSSH 9.9 <https://www.openssh.com/txt/release-9.9>

Co-authored-by: Mynacol <Mynacol@users.noreply.github.com>

nixos/modules/services/networking/ssh/sshd.nix

@@ -415,6 +415,8 @@ in
             KexAlgorithms = lib.mkOption {
               type = lib.types.nullOr (lib.types.listOf lib.types.str);
               default = [
+                "mlkem768x25519-sha256"
+                "sntrup761x25519-sha512"
                 "sntrup761x25519-sha512@openssh.com"
                 "curve25519-sha256"
                 "curve25519-sha256@libssh.org"