Update the pcluster site config to use pcluster 3.9.3

[eap]

As detailed in https://github.com/JCSDA-internal/jedi-tools/issues/372 we need to update the parallel cluster from 3.7.1 to 3.9.3. I've built a new site-config and updated the sections in the readme and the site-config assets for this new cluster.

Because this pull request only edited the site configs it was affected by the same test exclusion block noted in #1208 - I've moved the changes discussed there into this pull request ensuring that all required tests run.

[fabiolrdiniz]

Thanks, @eap!

[climbfuji]

There is a readthedocs build error - https://readthedocs.org/projects/spack-stack/builds/25158270/

[fabiolrdiniz]

I don't think it's related to this PR:

Running Sphinx v8.0.2
loading translations [en]... done
making output directory... done
Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`.
ERROR: Invalid value `None` in intersphinx_mapping['https://docs.python.org/']. Expected a two-element tuple or list.

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/spack-stack/envs/1210/lib/python3.11/site-packages/sphinx/cmd/build.py", line 332, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/spack-stack/envs/1210/lib/python3.11/site-packages/sphinx/application.py", line 277, in __init__
    self.events.emit('config-inited', self.config)
  File "/home/docs/checkouts/readthedocs.org/user_builds/spack-stack/envs/1210/lib/python3.11/site-packages/sphinx/events.py", line 100, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/spack-stack/envs/1210/lib/python3.11/site-packages/sphinx/ext/intersphinx/_load.py", line 127, in validate_intersphinx_mapping
    raise ConfigError(msg)
sphinx.errors.ConfigError: Invalid `intersphinx_mapping` configuration (1 error).

Configuration error:
Invalid `intersphinx_mapping` configuration (1 error).

But it seems that https://github.com/JCSDA/spack-stack/blob/develop/doc/source/conf.py#L203 should be changed to:

intersphinx_mapping = {'python': ('https://docs.python.org', None)}