text: Title: Change source distribution format from deprecated `zip` to `tar.gz`
  Body: Python source distribution format has been standardized to `tag.gz` by [PEP 625](https://peps.python.org/pep-0625/). We currently use `zip ` and PyPI has recently started sending deprecation warnings about that. Best to change the format before its support is removed altogether. In addition to change package itself, we need to make sure installation instructions are updated.
  
  This change doesn't affect normal users because `pip install robotframework` uses the wheel distribution and even when installing a downloaded source distribution, pip handles both formats fine. The change affects only users who download the source distribution and extract it, but that kind of usage is likely pretty rare. Those who actually want to see the source, typically clone the repository instead. Anyway, the change is somewhat backwards incompatible and needs to be mentioned in the release notes.