Just an update to anyone who follows this thread. According to MS support the option for .NET 8 (Early Access) is available under the drop-down menu of app service BUT the binaries are not deployed :(
Finaly, we got some feedback from MS support (see below).
"They're currently in the early staging process for sovereign clouds, which is expected to complete the rollout to all customers by February 1st."
Fingers crossed :)