Automated survey of latest available externals versions

[jcfreeman2]

At this week's Coordination Team meeting, Roland brought up - and Alessandro enthusiastically seconded - the idea of performing a survey of the current versions of the externals which we use as we wrap up v5.2 and move on to v5.3 development. He added that this is something which could be partially automated via a script which determines what the current version of each external in use is, and what the latest version available in Spack is. We can then use the results as a guide to look into situations where we're using old externals, want to see what the latest-greatest version of an external package is, etc.