Deprecate and remove clm4_5 physics

From the [SE meeting today](https://docs.google.com/document/d/1euq06v1pQOoN6NcGwdUqJsfgDNPqXzWB2cdMOJcydI8/edit?tab=t.0#heading=h.nq7hvqdxmrw): To ease our maintenance burden, before the release of CTSM6.0, we would like to remove `clm4_5` physics.

Definition of done:
- [x] Add as deprecated to IMPORTANT_NOTE, as well as the features that it uses
- [ ] Make notes elsewhere as needed about it being deprecated
- [x] Remove from tests
- [ ] Remove from compsets
- [ ] Remove from RTM testlist (https://github.com/ESCOMP/RTM/issues/68)
- [ ] Remove features from build-namelist and namelist-defaults
- [ ] Remove the code that uses those options
- [ ] Remove it from documentation
- [ ] Make sure it isn't mentioned anywhere

Start with IMPORTANT_NOTE and the existing compsets, RTM testlist, build-namelist, and namelist-defaults references to map every clm4_5 option. Then search the remaining tests, code, and documentation for mentions and run the relevant tests. Done means the physics, its options, compsets, RTM testlist, code, and documentation no longer contain unsupported clm4_5 references.

Written by the indexing model from the issue text.