Add clm6_0 physics option to CLM_PHYSICS_VERSION

[ekluzek]

As closure to the ctsm5.2.0 branch work we need to add a clm6_0 physics option. This is because with ctsm5.2.0 clm5_1 physics no longer makes sense, because tags will no longer be ctsm5.1.devXXX but tags after that point. With the CESM3 release CTSM will be ctsm6.0.0, so the physics version should be clm6_0.

Once, this is added, other components like: CISM, CAM, CMEPS will need to update compsets to point to clm6_0 rather than clm5_1, and remove clm5_1 from all compsets. The same is true of CLM we'll remove clm5_1 from our compsets and tests.

Also do a general look through and make sure README files and documentation in the code is reasonable.

[ekluzek]

@cacraigucar after we make the ctsm5.2.0 tag I'll make issues in CAM and CESM to update compsets for this.

[ekluzek]

For testing I'm just going to leave clm4_5, clm5_0 testing alone. Move all of the clm5_1 tests to clm6_0 and only keep a few key clm5_1 tests so that CAM and CESM shouldn't break until they remove it from their compsets.

[ekluzek]

It looks like the only testing that CAM and CESM have with clm5_1 are for the ne30np4.pg3 grid for SMS and ERP, on derecho with debug. So the clm5_1 testing can be very limited.

[ekluzek]

Need a 1850 BGC-CROP version for CESM and HIST SP version for CAM.

[ekluzek]

This came in with ctsm5.2.0 and #2417