terraform: use sensitive file resource

[v1v]

Motivation/summary

Use https://registry.terraform.io/providers/hashicorp/local/latest/docs/resources/sensitive_file when setting the ephemeral credentials.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

I created a feature branch and ran the benchmarks workflow:

• https://github.com/elastic/apm-server/actions/runs/12669908313

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