Fix discord and slack provider icon url

[RulerChen]

fix #48619

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[DjVinnii]

Isn't this image moved to airflow-core/src/airflow/ui/public?

[RulerChen]

Isn't this image moved to airflow-core/src/airflow/ui/public?

I only see pin_32.png in there. I think the image size is a little bit different, or do I need to push pin_100.png there?

[RulerChen]

Hi, @eladkal. I have replaced all the URLs I found with the correct ones. Please take a look again. Thanks!

[boring-cyborg]

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