Remove Gaea C5 platform support

[Copilot]

Description

Gaea C5 is no longer supported. This PR removes all references to gaeac5 while preserving Gaea C6 (gaeac6) and all other platform configurations.

Changes:

• Platform detection - Removed gaea5[1-8] hostname patterns and /gpfs/f5 path detection from ush/detect_machine.sh
• Module loading - Removed gaeac5-specific module setup logic from ush/module-setup.sh
• Build system - Removed gaeac5 FIX_DIR path from sorc/link_workflow.sh
• CI infrastructure - Removed gaeac5 from Jenkins node mappings, GitLab runner configurations, and Java agent launch scripts
• Test cases - Removed gaeac5 from skip_ci_on_hosts in 17 CI test YAML files
• Configuration files - Deleted dev/workflow/hosts/gaeac5.yaml and dev/ci/platforms/config.gaeac5

Verification:

• Zero gaeac5 references remain (grep verified)
• Gaea C6 preserved (55 gaeac6 references intact)
• Other platforms unaffected (hera, orion, hercules, wcoss2, ursa)

Type of change

• Maintenance (code refactor, clean-up, new CI test, etc.)

Change characteristics

• Is this change expected to change outputs (e.g. value changes to existing outputs, new files stored in COM, files removed from COM, filename changes, additions/subtractions to archives)? NO
• 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?

Automated verification:

• grep -r "gaeac5" returns 0 matches
• find . -name "*gaeac5*" returns 0 files
• All supported platforms (hera, orion, hercules, wcoss2, ursa, gaeac6) retain proper references

Checklist

• Any dependent changes have been merged and published
• 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
• New and existing tests pass with my changes
• This change is covered by an existing CI test or a new one has been added
• Any new scripts have been added to the .github/CODEOWNERS file with owners
• I have made corresponding changes to the system documentation if necessary

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove references to Gaea C5</issue_title>
<issue_description>### What task needs to be completed?

Gaea C5 is no longer supported in the global workflow. References to the system should be removed.

Definition of Done

• A grep and find for gaeac5 returns no matches
• A grep and find for gaea only returns references to Gaea C6
• Operation on other systems is unaffected</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove references to Gaea C5 NOAA-EMC/global-workflow#4491

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