Add OpenTofu CI/CD components and deprecate Terraform CI/CD templates
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## Context
GitLab ships with [a series of CI templates that reference a GitLab built container with Hashicorp Terraform included](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Terraform.latest.gitlab-ci.yml). The community is already working on [an opentofu based image](https://gitlab.com/gitlab-org/terraform-images/-/issues/114).
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
### A note for Terraform users
GitLab does not plan to remove support for Terraform. We are deprecating the minimal necessary to comply with the BuSL license, this means the CI/CD templates that were shipped with GitLab. All the other functionality is expected to remain.
Sometime in the future, we might need to pick Terraform or OpenTofu as the they expected to diverge. This is not needed yet, and we don't want to force anyone to change their preferred IaC tool. This should be your choice, and we want to support you the best we can.
## 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?
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