Drop old code

[Yang-33]

Starting from v8.0.0, we have automated code generation. It's been a year since then. To allow users to migrate, we've kept the old code for now, but we plan to remove it soon.

If users want to use the latest features, they should migrate their code. If the latest features are not needed, there's no issue with not upgrading the version. However, please note that older versions will not be supported.

Task list to accomplish this:

1. Add @deprecated to all old code and specify the migration path.
2. Increase the patch version and publish it.
3. Document the migration method somewhere.
4. Remove the old code.
5. Increase the major version and publish it.

A tool for automatic code migration will not be provided. Of course, if you read this and you're interested to contribute to this project and our community, you can write a migration tool and contribute.