Replace Terraform 1.3.8 with OpenTofu 1.9.1, bump go to 1.24

[thentenaar]

This is a pretty hefty PR due to all the file replacements, and I'm not sure if there's interest, but in brief the changes are:

• terraform 1.3.8 -> tofu 1.9.1
• opa 0.69.0 -> 1.4.2
• Update to go 1.24
• Fix various instances of format strings being non-const

go test ./... is quite happy with the changes.

[CLAassistant]

All committers have signed the CLA.

[rontalx]

Hi @thentenaar, thanks for the contribution proposal.
Could you please share some background context to the proposed changes?

[thentenaar]

Hi @rontalx, thanks for the comment. I've been looking into the policy-engine, wanted to use it with something more recent than terraform 1.3.x (tofu providing as close as you can get to a license-compatible update beyond 1.5.7) with updated dependencies, etc. I'd noticed it hadn't been updated, and figured to hack together a quick PR to gauge interest in such an update.

It's likely the code brought in from tofu could be pared down, and I'd be willing to whittle away at it if there's interest.