Remove Clm51 references throughout or change to Clm60

[slevis-lmwg]

Description of changes

TODO list appears in the corresponding issue #2379.

Specific notes

Contributors other than yourself, if any:
@ekluzek

CTSM Issues Fixed (include github issue #):
Fixes #2379.

Are answers expected to change (and if so in what way)?
No.

Any User Interface Changes (namelist or namelist defaults changes)?
Removing Clm51 references throughout.

Does this create a need to change or add documentation? Did you do so?
Yes.

Testing performed, if any:
On derecho:

• PASS make black and make lint
• PASS ./run_ctsm_py_tests -s and ./run_ctsm_py_tests -u
• PASS ./build-namelist_test.pl
• OK aux_clm without mods to the submodules, yet

[slevis-lmwg]

OK aux_clm on derecho before rebasing from master to the cesm3_0_beta04_changes branch.

After rebasing to the cesm3_0_beta04_changes branch:
OK aux_clm on derecho
OK aux_clm on izumi
PASS make black and make lint
PASS ./run_ctsm_py_tests -u and ./run_ctsm_py_tests -s
PASS ./build-namelist_test.pl (still running, but I expect this to pass)

This PR is ready to merge. I need to wait for @ekluzek's review and approval, especially due to the potential for conflicts as mentioned in this post.