Add minimum Windows requirements

Hello!

Prior to downloading ActivityWatch on Windows, it would be great if the minimum Windows requirements were mentioned on the download page(s), so as to let users know whether their OS will support ActivityWatch.

https://activitywatch.net/, https://activitywatch.net/downloads/#windows, https://docs.activitywatch.net/en/latest/getting-started.html do not list what are the minimum requirements to run ActivityWatch on Windows.

Given that python 3.9 is used, and according to [python's downloads page for Windows](https://www.python.org/downloads/windows/), this would indicate that the minimum Windows version required to use ActivityWatch is Windows 8.

- [x] I have searched the [issues](https://github.com/ActivityWatch/activitywatch/issues) of this repo and believe that this is not a duplicate.
- [x] I have searched the [documentation](https://docs.activitywatch.net/en/latest/) and believe that my question is not covered.