The best solution is for YOU to actually look at the dates when the catalog files were created or last used, and determine which is the most recent catalog file. (The catalog files end with .LRCAT if you weren't sure)
But to be 100% certain, you need to be sure that this newest LR catalog file shows all of the photos that you are expecting it to have. The upgraded catalog file should have the same number of photos as the LR 5 catalog at the time you did the upgrade. If that's the case, you can keep the newest catalog file and archive or delete the older one. You can delete the previews of the older one as well.
Did you see that there are now two Lr icons in your dock? If you hover over the right-most icon it should say Lightroom Classic and if you hover over the left-most icon it should say Lightroom 5. If you open Lr Classic from either the dock or the Creative Cloud Manager at the top of your screen you will be prompted to update your catalog. When you choose to let Lr update, a second Lr catalog will be created and it will have a (-2) added to the file name: Lightroom Catalog-2.lrcat. The Lightroom folder as you have shown in your screen shot will show both .lrcat and -2.lrcat files.
Once you are sure that your Lr Classic file is working as expected (after you update your catalog) you can remove the old lrcat (v.5) and associated previews and data files. The new preview and data file will also have a -2 so you will be able to differentiate them. If it has a -2 keep it in the folder.
Hope that is helpful.
Thank you all for your replies and help on this one.