v6: Add .GEOSdefaults.yaml, add FV3 GC to develop list

[mathomp4]

This adds a .GEOSdefaults.yaml file for use with upcoming linkbcs.py work (see GEOS-ESM/GEOSgcm_App#823).

It also adds GEOS_Util and, temporarily, FVdycoreCubed_GridComp to the default mepo develop list.

The changelog is reconciled with published v6 tags and notes that v6.6.0 was skipped.

[mathomp4]

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