Update documentation w/switch to spack-stack

[gspetro-NOAA]

Description

The WM docs still indicate that the WM is using HPC-Stack. The docs require an update (preferably ahead of the SRW App v2.2.0 release) to indicate that the WM has switched to spack-stack.

To Reproduce:

The old reference to HPC-Stack as the supported stack can be found here: https://ufs-weather-model.readthedocs.io/en/latest/BuildingAndRunning.html#prerequisite-libraries

Additional context

N/A

Output

N/A