Rework physics versions used in our System tests (least clm4_5, midlevel clm5_0, and most clm6_0)

With #2378 coming in we need to look at our test lists and revamp them in view of the CESM3 release. Mainly I think most tests should be clm6_0, but some still for clm5_0 and clm4_5.

With #2379 we also should have minimal testing for clm5_1. But, we can only completely remove it when CAM and CESM have moved to remove clm5_1 in their compsets.

Maybe it's time to drop *some* of the clm4_5 testing with a new physics version available? I think the testing should scale up from clm4_5 with more for clm5_0 and then the most for clm6_0.