Update new machine (Gaea-C6) and upgraded machine (Gaea-C5)

[RatkoVasic-NOAA]

Summary

Update documentation and yaml files for affected machines: Gaea-C5 and Gaea-C6

Testing

Already installed spack stacks on these machines.

Applications affected

All using spack stack on Gaea

Systems affected

Gaea C5 and C6

Issue(s) addressed

#1368

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.

[rickgrubin-noaa]

LGTM -- approved.