[filebeat][gcs] - Fixed the source of max workers in scheduler

[ShourieG]

• Bug

What does this PR do?

A small code error got introduced with the PR: #35605, it went unnoticed in the PR reviews because how small and similar that change was. It's not an error as such but can cause logical issues because of how the config is parsed. This has been fixed in the back-ports of the previous pr manually.

Why is it important?

The max worker count being used from the config directly creates a scenario where it only used the global config values
and not the bucket level config values stored in the source object.

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-developer.next.asciidoc.

Author's Checklist

• [ ]

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @ShourieG? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[kcreddy]

LGTM

[elasticmachine]

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• Start Time: 2023-06-09T10:32:39.204+0000

• Duration: 73 min 46 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2989
Skipped	177
Total	3166

💚 Flaky test report

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