Promote unable to terminate warning to logger.WARNING (#3574)

Even if the subsequent SIGKILL works, this is an exceptional circumstance
that should be logged.

parsl/executors/high_throughput/executor.py

@@ -832,7 +832,7 @@ def shutdown(self, timeout: float = 10.0):
         try:
             self.interchange_proc.wait(timeout=timeout)
         except subprocess.TimeoutExpired:
-            logger.info("Unable to terminate Interchange process; sending SIGKILL")
+            logger.warning("Unable to terminate Interchange process; sending SIGKILL")
             self.interchange_proc.kill()
 
         logger.info("Closing ZMQ pipes")