catch cancled task exception in notif

BTCPayServer/Controllers/NotificationsController.cs

@@ -91,6 +91,10 @@ namespace BTCPayServer.Controllers
                     await Task.Delay(2000, cancellationToken);
                 }
             }
+            catch(TaskCanceledException)
+            {
+                // ignored
+            }
             finally
             {
                 subscription?.Dispose();