keep tasks running on shutdown (they could be shutdown handlers)

nicegui/nicegui.py

@@ -64,8 +64,6 @@ def handle_shutdown() -> None:
     globals.state = globals.State.STOPPING
     for t in globals.shutdown_handlers:
         safe_invoke(t)
-    for t in background_tasks.running_tasks:
-        t.cancel()
     globals.state = globals.State.STOPPED