fix(terraform): use key_schema for DynamoDB GSIs

[edersonbrilhante]

Description

Updates the GitHub global lock DynamoDB table configuration to replace deprecated hash_key arguments on global secondary indexes with key_schema blocks.

This keeps the existing GSI partition keys unchanged (workflow_run_id and workflow_run_attempt) while aligning the module with the AWS provider 6.x DynamoDB schema. The table-level primary key remains hash_key = "lock_id" because key_schema is only supported for the GSI configuration in this provider schema.

Technical reference: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/dynamodb_table.html

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass