[feature-request] Use robot to update requirements.txt

`requirements.txt` is introduced in #9785. It should be the result of `pip freeze`.
However, as time goes by, these files are edit manually when we bump package. Some dependencies are out-of-date and may have version conflict (see #28017).

I think we use a robot to improve this:
1. Developers only need to modify `setup.py`.
2. The robot creates `requirements.txt` using `pip freeze` and commits it to the pull request.
3. (The robot may also periodically update `requirements.txt`.)