testsuite: note job-info.update-lookup is legacy

Problem: The job-info.update-lookup RPC target has been deprecated.
It would be wise to note this in some legacy tests.

Add note in t/job-info/update_lookup.c.

Author: chu11

t/job-info/update_lookup.c

@@ -8,6 +8,10 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+/* Note that the job-info.update-lookup RPC target is deprecated.
+ * This is to test legacy behavior.
+ */
+
 #if HAVE_CONFIG_H
 #include "config.h"
 #endif