new release v12.2.0

[jbathegit]

What needs to be done:

• increment number in VERSION file
• update release notes, in both GitHub and in ReleaseNotes.md
• move the existing html to previous_versions/v12.1.0, and add a new reference link for it within user_guide.md
• generate docs and update gh-pages branch with the new html
• after the release is done through GitHub, download the resulting v12.2.0.tar.gz file, do a shasum -a 256 on that file to get a hashcode, then add the new version and hashcode to spack/package.py, then push a new commit to the develop branch
• open a new issue at https://github.com/NOAA-EMC/WCOSS2-requests for installation on WCOSS2