split terraform and CI scaffolding into separate repos

[jku]

Filing for visibility:

• this repository currently contains various resources for testing in CI as well as reusable terraform modules, this is confusing as these two things are not that closely related
• @haydentherapper has already created a new repository https://github.com/sigstore/terraform-modules for the terraform modules (with git history from this repo)
• this repository should remain the CI scaffolding repo

We should aim to finalize the split ASAP to avoid confusion and diverging content:

• switch public good instance to use terraform-modules
• inform any other known scaffolding terraform users (sigstore/rekor at least)
• delete terraform modules in this repo
• update README (there's only one minor mention of terraform)