OECDRegionalDemography_Population| Fixed missing data issue

[Ashwani725]

2024 onwards source data mapping has changed due to which data is not refreshed. Fixed this by adding the missing mapping in the schema
files changed are: OECD_population_stat_vars.mcf, columns.py, manifest.json, golden_data/, validation_config.json

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[gemini-code-assist]

Code Review

This pull request introduces new statistical variables for age groups 20-64, 25-54, and 55-64, including gender-specific breakdowns, and updates the associated column mappings and golden data. Additionally, a new validation configuration file is added to the manifest. The reviewer recommended updating the rule_id in the validation configuration for consistency and noted that the expected test files must be updated to reflect these schema changes to prevent unit test failures.