Highlight the preference to use Terraform HTTP vars

What does this MR do?

Highlight the preference to use Terraform HTTP vars

    It's been a common error to use terraform init -backend-config=password=$CI_JOB_TOKEN while
    using the GitLab Terraform template. This is a problem since the CI_JOB_TOKEN gets cached during
    terrform plan, then gets forwarded to the terraform apply job. Although, for the second job
    the token will be a different one, thus resulting in an authentication error. Using env vars
    should help with that.

Related issues

Closes #346075 (closed)

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