following the Twilio documentation here: https://www.twilio.com/docs/notify/configure-android-push-notifications#step-5-upload-your-api-key-to-twilio I just noticed that firebase changed its Server Key method to use its Api.
While on Twilio documentation I see this:

How can I obtain or generate the new key Twilio needs?
I believe that you need to use the Server key listed under Cloud Messaging API (legacy) for now.