Fix documented Python version for venv --upgrade-deps (GH-22113)

Fixes incorrect Python version added for `venv` `--upgrade-deps` in #13100. This feature was added in Python 3.9 not 3.8.

Relates to:

- 
- https://github.com/python/cpython/commit/1cba1c9abadf76f458ecf883a48515aa3b534dbd

Automerge-Triggered-By: @vsajip