DEP/CI: Pin flake8-rst version
- [x] closes #23665

Pins version of `flake8-rst` to `0.4.2`. 

* Newer versions of this plugin will cover more code-blocks within the documentation. Thus CI will fail when new version is released to conda-forge. 
* Devs using pip already get issues as version `0.4.3` is released to PyPI.

`flake8-rst` is getting upgraded shortly to a version `>0.4.3`. I'm currently updating the doc so it becomes compliant with that newer version, therefore I don't open issues to fix the doc for version `0.4.3`. 
