Enabled the feature flag ci_prevent_file_var_expansion to true

What does this MR do and why?

Feature Flag Issue link: #414583 (closed)

The feature flag ci_prevent_file_var_expansion_downstream_pipeline has been enabled in production and now the MR sets the default to true in code.

Since 15.7, file type variables should expand to the filename instead of the file content. However, in child pipelines, the file content is still expanded. More context in the issue: #390252 (closed)

MR acceptance checklist

• Enabled ci_prevent_file_var_expansion_downstream_pipeline in prod

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