fix: serialization of status list

[itsyaasir]

Description of change

This PR addresses the serialization issue with the status_list_index field, which was incorrectly serialized in the StatusListEntry2021 type.

Links to any relevant issues

Fixes the wrong index type in the source code; Link here

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Describe the tests that you ran to verify your changes.
Make sure to provide instructions for the maintainer as well as any relevant configurations.

Change checklist

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• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[itsyaasir]

@UMR1352 Correct serialization applied.