Does anyone know where I can find the Server Key shown in this picture? The problem is that Firebase slightly changed the UI and now I don't know where to find it.

Firebase cloud messaging is now shifted to API (V1)
So to get the serverkey you should enable the cloud messaging API (Legacy).
Follow the steps:
Click on the three dots and click on manage Api in google cloud console

Click on enable.

3.Now redirect back to the main page and refresh the page.
Now you have your serverkey
Hope it helps!
From Firebase Cloud Messaging API (V1), we have to enable the cloud messaging service.
Reference - https://documentation.onesignal.com/docs/generate-a-google-server-api-key
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