[8.13](backport #38421) Make log messages in the file scanner less noisy

[mergify]

• Demote warning about empty files to the Debug level
• Demote warnings about files too small to the Debug level
• Improve wording, so it does not sound like an error

Checklist

• 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.

How to test this PR locally

cd ./filebeat/input/filestream
go test

Related issues

• Relates Fix empty file edge case #36076
• Relates Add new fingerprint file identity #35734

  ---

  This is an automatic backport of pull request Make log messages in the file scanner less noisy #38421 done by Mergify.

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[elasticmachine]

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