Migrate to MongoDB 8.2

### Problem description

Our system currently relies on MongoDB 6, which has already reached its end-of-life and no longer receives security updates or fixes. We must transition to MongoDB 8.2 to ensure long-term stability and security.
We have both existing users who will require a smooth database migration, and new users who should start with MongoDB 8.2 or newer.
The upgrade is motivated not only by EOL concerns but also by the significantly improved performance and additional new capabilities offered in MongoDB 8.x, which can help us further evolve and optimize the product.

### Requirements

- Switch the system to use MongoDB 8.2 (or a later 8.x version).
- Provide a reliable migration path for existing users so their data and system functionality remain intact after upgrading.
- Ensure new users deploying the system for the first time start with MongoDB 8.2+.
- Update all documentation to reflect the new MongoDB version and the required migration steps.
- Confirm that the upgraded system behaves the same as before the version change and that all tests pass without errors.

### Definition of done

- [ ] Functionality is implemented as specified above.
- [ ] New requirements are documented.

### Acceptance criteria

- Existing users can migrate from MongoDB 6 to 8.2 following the documentation, and the system continues to function normally afterward.
- New users can deploy the system cleanly using MongoDB 8.2+.
- Documentation updated accordingly.
- All features behave consistently with the pre-upgrade version.
- All tests run successfully with zero errors.
- No configuration, code, or documentation references MongoDB 6.