Make terraform templates secure by default

What does this MR do and why?

Describe in detail what your merge request does and why.

With GitLab-managed Terraform state, Terraform state files are stored as artifacts. These files can contain sensitive data, and having them available in public artifacts is a security concern.

Our own documentation recommends people manually change this setting:

We also strongly recommend that you disable public pipelines by setting the artifact’s public flag to false (public: false)

We should ensure we are using our own best practices in any templates we provide, to make it easy for everyone to be secure by default.

This MR adds the recommended flag to the baseline Terraform templates.

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