Updating Windows contributor requirements to run Windows 10

We recently ran into a situation where git errored during a `checkout` due to a file path that was too long. https://github.com/microsoft/TypeScript/pull/35711#issuecomment-570730960 To get around this, we can run ``` git config core.longpaths true ``` However, it seems that this option requires Windows users to run Windows 10. It doesn't affect other users. Everyone on the core team who's on Windows is basically on the latest version, and we don't have reason to believe at the moment that this affects many external contributors. As a result, we will likely turn on the flag and adjust appropriately if this becomes an issue.