[PJLink] forget threadpool after it was shutdown (#16119)

This is an issue if the user cancels/restarts discovery scans, new tasks were added to the shutdown threadpool.
With this PR, a new threadpool will be created on the next scan in getExecutorService().

Signed-off-by: Nils Schnabel <github@to.nilsschnabel.de>

bundles/org.openhab.binding.pjlinkdevice/src/main/java/org/openhab/binding/pjlinkdevice/internal/discovery/AbstractDiscoveryParticipant.java

@@ -97,6 +97,7 @@ protected synchronized void stopScan() {
             Thread.currentThread().interrupt(); // Reset interrupt flag
         }
         executorService.shutdown();
+        this.executorService = null;
     }
 
     public static ThingUID createServiceUID(String ip, int tcpPort) {