examples/ecs-alb: Replace template_file data sources

[democracytoday]

Update the examples to use the "new" templatefile() function.

The template provider has been deprecated since 0.12 and is not available on all platforms, such as arm64.

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Relates #22739

Output from acceptance testing: N/A, this is only a documentation update.

[github-actions]

Welcome @democracytoday 👋

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[justinretzolk]

Looks good to me! Thank you for your contribution! I'll get this merged in once the current release process finishes up and the checks have completed! 🚀

[justinretzolk]

Update: looks like the checks didn't like some of the formatting of main.tf, so I pushed a commit after running terraform fmt.

[democracytoday]

Great, thanks for handling the formatting changes!

[github-actions]

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