Move the Terraform template to `rules` syntax

Problem to solve

As a Platform engineer, in order to build upon the stock GitLab Terraform CI templates, I need easy inclusion and customizations. The rules syntax allows the required flexibility, but is not supported with the current template.

Proposal

The rules syntax is the recommended approach for condition job running specification. Migrate the latest Terraform template that ships with GitLab from the only/except/when to the recommended rules syntax.