Add ACCESS-ESM1.6-specific deployment information

[CodeGat]

Background

Given ACCESS-ESM1.5 can be used as a base for ACCESS-ESM1.6, cherry pick HEAD of ACCESS-ESM1.5 in a new PR branch, and add the ACCESS-ESM1.6-specific parts to that PR.

Things TODO

• Add in secrets and vars as in the README. See https://github.com/ACCESS-NRI/ACCESS-ESM1.6/settings/secrets/actions if able
• Cherry pick HEAD of ACCESS-ESM1.5: Used ACCESS-NRI/ACCESS-ESM1.5@a17379f. This is on https://github.com/ACCESS-NRI/ACCESS-ESM1.6/tree/1-initial-spack-yaml
• Open PR when we have an ESM1.6 SBD in https://github.com/ACCESS-NRI/spack-packages
• Add ACCESS-ESM1.6-specific changes!

Pinging @dougiesquire so he has visibility on the progress.