[8.x](backport #42019) x-pack/filebeat/input/awss3: fix document ID construction when using csv decoder

[mergify]

Proposed commit message

See title.

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• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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This is an automatic backport of pull request #42019 done by [Mergify](https://mergify.com).

[botelastic]

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.x/pr-42019 upstream/mergify/bp/8.x/pr-42019
git merge upstream/8.x
git push upstream mergify/bp/8.x/pr-42019

[mergify]

This pull request has not been merged yet. Could you please review and merge it @efd6? 🙏