The branch jlaur:14222-energidataservice is targeting the main branch, i.e. openHAB 4.0. It requires Java 17 as well as Gson 2.10. Therefore you cannot build it for 3.4.
I have backported it to a private branch. You can find a JAR that can be used directly with openHAB 3.4 here: Energi Data Service Binding [3.4.0.0;4.0.0.0)
You can also install it straight from the UI under Community Add-ons.