Change the Terraform.latest CI template to use an opentofu based image

Context

GitLab ships with a series of CI templates that reference a GitLab built container with Hashicorp Terraform included. The community is already working on an opentofu based image.

Hashicorp switched to BuSL template in August 2023. Our internal legal analysis suggests that

• we can continue offering all our features as is on GitLab SaaS and Dedicated
• we are not allowed to ship the Terraform CI templates with GitLab Self-Managed

Proposals

Proposal 1

• Continue shipping the Terraform CI templates for SaaS only
• Replace them with OpenTofu template for Self-Managed.

Proposal 2

Completely remove the Terraform CI templates from SaaS and Self-Managed, and replace them with OpenTofu templates.

Replacing the templates

1. We should create new CI templates
2. We should deprecate the Terraform templates and set them up for removal
   • Instead of a pure purge, we should extend the Terraform templates with a job that educates the user how to switch
3. Drop all (4) the Terraform templates to avoid infringing Hashicorp license.
   • Could we do this for Self-Managed only?