Version listing for documentation

[AlexanderRichert-NOAA]

Let's create a script that can be run for the unified env for each release and spit out a list of packages of interest (maybe use 'spack dependents' to get direct dependencies for our *-env modules). This can then be put on the wiki in a table with the most recent four versions so we'll have one very easy place to point to when people ask what versions are in a given release.