Updated link formatting for clarity in changelog.

* feat: add device_id to flags request payload

Add device_id parameter to all feature flag methods, similar to how
distinct_id is handled. The device_id is included in the flags request
payload sent to the server.

- Add device_id parameter to Client methods and module-level functions
- Add context support via set_context_device_id() for automatic fallback
- Add tests for explicit device_id and context-based device_id
- Bump version to 7.6.0