[8.x](backport #40937) [filebeat][gcs] - Simplified state checkpoint calculation

[mergify]

Type of change

• Enhancement

Proposed commit message

• WHAT: Simplified state checkpoint calculation
• WHY: The existing state checkpoint calculation logic was convoluted and depended on unreliable flag logic, then it has been simplified so that if errors occur they are become simpler to debug.

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.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Simplify GCS input state calculation logic #40878
• Relates [filebeat][azure-blob-storage] - Simplified state checkpoint calculation #40936

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #40937 done by [Mergify](https://mergify.com).

[botelastic]

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[mergify]

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