For ctsm5.4: WhatsNewInCTSM5.4.md

[slevis-lmwg]

Description of changes

I used WhatsNewInCTSM5.3.md as the starting point for WhatsNewInCTSM5.4.md and started modifying in subsequent commits.

Specific notes

Contributors other than yourself, if any:
ctsm software engineering group

CTSM Issues Fixed (include github issue #):
Resolves #3003

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

Any User Interface Changes (namelist or namelist defaults changes)?
No

Does this create a need to change or add documentation? Did you do so?
This PR updates WhatsNewInCTSM5.3.md to WhatsNewInCTSM5.4.md

[slevis-lmwg]

Do not replace the 5.3 with the 5.4, just add.

Also, I should add the following from Sam R's email on 2025/4/15:

the ctsm5.3.040 tag we just published has some implications for how you use the CTSM Python tools.

Specifically, the new ctsm_pylib conda environment will be incompatible with our tools from before ctsm5.3.040 and vice versa. If you already have a copy of the ctsm_pylib conda environment installed, we thus suggest keeping that around under a different name. Here is the suggested command for doing this in a local copy of ctsm5.3.040 or later:

./py_env_create -r ctsm_pylib_old

This will first rename your existing ctsm_pylib to ctsm_pylib_old, then install the Python 3.13.2 version as ctsm_pylib. (If you’re not sure whether you already have ctsm_pylib installed, you can still use that command—it will just skip the rename step.)

Information about additional py_env_create options—including how to install a fresh copy of the old conda environment—can be found like so:

./py_env_create --help

[ekluzek]

Add the 5.4 version to the top level, and move the 5.3 version into the doc subdirectory.

[samsrabin]

The ctsm software group is updating the Google Docs version

[slevis-lmwg]

@slevis-lmwg to email ctsm-software a request to look at the google doc in prep. for the 5.4 release