replace deprecated asyncio.get_event_loop()

`asyncio.get_event_loop()` was deprecated in Python 3.10, so we should use `asyncio.get_running_loop()` instead.

docs/source/high-level-service/index.rst

@@ -90,4 +90,4 @@ If any file descriptors are sent or received (DBus type ``h``), the variable ref
 
         await bus.wait_for_disconnect()
 
-    asyncio.get_event_loop().run_until_complete(main())
+    asyncio.get_running_loop().run_until_complete(main())