Deprecate AES algorithm for DNS cookies

The AES algorithm for DNS cookies was being kept for legacy reasons,
and it can be safely removed in the next major release.  Mark is as
deprecated, so the `named-checkconf` prints a warning when in use.

lib/isccfg/check.c

@@ -1561,6 +1561,10 @@ check_options(const cfg_obj_t *options, const cfg_obj_t *config,
 	(void)cfg_map_get(options, "cookie-algorithm", &obj);
 	if (obj != NULL) {
 		ccalg = cfg_obj_asstring(obj);
+		if (strcasecmp(ccalg, "aes") == 0) {
+			cfg_obj_log(obj, logctx, ISC_LOG_WARNING,
+				    "cookie-algorithm 'aes' is deprecated");
+		}
 	}
 
 	obj = NULL;