Create externals v2.3

[jcfreeman2]

It's now been almost year since externals v2.2 was released, and particularly since there will be some quiet over the holiday season, now will be a good time into looking into which externals packages' versions we can bump for an externals v2.3 release.

Note that I mentioned this at the Coordination Workshop last week, and in particular made the point that if people feel strongly about particular versions they should let me know.