The `latest` Terraform version silently rewritten to 1.5.7, downgrading workspaces by ~10 minor versions

[tdabasinskas]

When a TFC workspace's terraform-version is latest, spacemk generate rewrites it to 1.5.7 with TERRAFORM_FOSS:

if terraform_version == "latest":
    # KLUDGE: Stick to the latest MPL-licensed Terraform version for now
    terraform_version = "1.5.7"
    terraform_workflow_tool = "TERRAFORM_FOSS"

TFC's latest today resolves to Terraform 1.15.x, so this silently downgrades migrated stacks ~10 minor versions. Consequences:

• State written by 1.6+ may not be readable by 1.5.7 → first plan after migration fails.
• Modules with required_version >= 1.6 fail on terraform init.

At minimum, it would be good to see a WARNING on that.