This pull request reorganizes and updates many country-specific financial reporting modules to align with Odoo's newer application programming standards. The change should improve maintainability and consistency across localized reports, with limited direct impact on day-to-day business users.

Description of the issue/feature this PR addresses:
- [l10n_*_reports]migrate to new API

Task:https://www.odoo.com/web#id=27254&view_type=form&model=project.task&action=333&active_id=131&menu_id=4720
Pad:https://pad.odoo.com/p/r.4f2026b592a0937a5a94828258e79a6a

The Belgian reports module has been reorganized and migrated to Odoo's newer development framework. This should make the Belgian VAT and financial reporting features easier to maintain and evolve, with limited direct impact on day-to-day users.