You can update the Fabric loader version in the json file in the version folder.
I don't think the java server thing is related to Aristois. I am able to run the mod with 21. You can try changing it in the installation settings though.
@joutico777 use ViaFabricPlus version 4.0.4, not the one aviable on the Aristois addons feature. Ive tried it and it works, you can play 1.21.5 servers using Aristois 1.21.4