Where can I find the list of icons (both images and names) for version 12.0.0?
The icons can be found in the showcase. However, the current showcase that's online is the one of the current PrimeFaces version. The showcase uses PrimeIconsView.java to get the available icons for the showcase. So you can simply have a look at that file 12.0.0: https://github.com/primefaces/primefaces/blob/12.0.0/primefaces-showcase/src/main/java/org/primefaces/showcase/view/misc/PrimeIconsView.java
There you'll find that it uses PrimeIcons 6.0.1 and that it loads the names from: https://raw.githubusercontent.com/primefaces/primeicons/6.0.1/selection.json
You can also download the PrimeFaces repository tagged with 12.0.0 and build and run the showcase as described in the readme: https://github.com/primefaces/primefaces/tree/12.0.0/primefaces-showcase