use :lock:  for closed issue instead of :exclamation: so meaning is better conveyed

before:

![Screenshot from 2021-09-24 17-27-04](https://user-images.githubusercontent.com/394074/134635320-ef6d335e-7e4d-4cef-a65f-ae50a36a10f7.png)

after:

![Screenshot from 2021-09-24 17-26-42](https://user-images.githubusercontent.com/394074/134635367-aa26a3aa-6485-495a-8711-1ce145d6a92f.png)