TMSS-780: block subtask scheduling when obsolete

SAS/TMSS/backend/src/tmss/tmssapp/subtasks.py

@@ -1096,6 +1096,9 @@ def check_prerequities_for_scheduling(subtask: Subtask) -> bool:
     if subtask.state.value != SubtaskState.Choices.DEFINED.value:
         raise SubtaskSchedulingException("Cannot schedule subtask id=%d because it is not DEFINED. Current state=%s" % (subtask.pk, subtask.state.value))
 
+    if subtask.obsolete_since is not None:
+        raise SubtaskSchedulingException("Cannot schedule subtask id=%d because it is marked as obsolete since %s" % (subtask.pk, subtask.obsolete_since))
+
     for predecessor in subtask.predecessors.all():
         if predecessor.state.value != SubtaskState.Choices.FINISHED.value:
             raise SubtaskSchedulingException("Cannot schedule subtask id=%d because its predecessor id=%s in not FINISHED but state=%s"