[cbus] fix exception from threadpool at startup (openhab#11846)

* update to use executorservice to fix crash caused by change in underlying threadpool implementation

* Remove explicit cast

Signed-off-by: John Harvey <john.p.harvey@btinternet.com>
Signed-off-by: Nick Waterton <n.waterton@outlook.com>

bundles/org.openhab.binding.cbus/src/main/java/org/openhab/binding/cbus/internal/CBusThreadPool.java

@@ -14,7 +14,7 @@
 
 import java.util.HashMap;
 import java.util.Map;
-import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.ExecutorService;
 
 import org.eclipse.jdt.annotation.NonNullByDefault;
 import org.eclipse.jdt.annotation.Nullable;
@@ -45,10 +45,10 @@ protected synchronized CGateThreadPoolExecutor CreateExecutor(@Nullable String n
     }
 
     public class CBusThreadPoolExecutor extends CGateThreadPoolExecutor {
-        private final ThreadPoolExecutor threadPool;
+        private final ExecutorService threadPool;
 
         public CBusThreadPoolExecutor(@Nullable String poolName) {
-            threadPool = (ThreadPoolExecutor) ThreadPoolManager.getPool("binding.cbus-" + poolName);
+            threadPool = ThreadPoolManager.getPool("binding.cbus-" + poolName);
         }
 
         @Override