Produce greenhouse gas concentration ancillaries for the ACCESS-ESM1.6 `ScenarioMIP` h and vl experiments

[penguian]

See https://input4mips-cvs.readthedocs.io/en/latest/dataset-overviews/greenhouse-gas-concentrations/ and ACCESS-NRI/access-esm1.6-configs#791 as well as #86

Update an existing branch of https://github.com/ACCESS-NRI/access-esm1.6-configs in order to update atmosphere/namelists with greenhouse gas concentrations, initially for the H and VL scenarios of ScenarioMIP. The update method is based on #86.

Unresolved question: In the historical configuration, CLIM_FCG_YEARS contains the years 1850 to 2023. For ScenarioMIP, should CLIM_FCG_YEARS contain only 2025 to 2100?