Add/update config files for Gaea and check existence before sourcing config files in generate_workflows.sh

[DavidHuber-NOAA]

Description

This PR adds, renames, and updates values in ci/platforms/config.gaeac5/6 to allow generate_workflows.sh to pick up on a default HPC_ACCOUNT and removes STMP from those files to prevent issues with the setup scripts.

Related to #3261

Type of change

• Bug fix (fixes something broken)

Change characteristics

• Is this a breaking change (a change in existing functionality)? NO
• Does this change require a documentation update? NO
• Does this change require an update to any of the following submodules? NO

How has this been tested?

Ran generate_workflows.sh on Gaea-C6

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have documented my code, including function, input, and output descriptions
• My changes generate no new warnings

[AnilKumar-NOAA]

These changes looks great! Thanks @DavidHuber-NOAA

[KateFriedman-NOAA]

Looks good, thanks @DavidHuber-NOAA ! Will likely change some settings (e.g. GFS_CI_ROOT) once we get established to run CI tests on Gaea C6. These settings are good for now though.