When getting system directories for Windows, we currently use
SHGetFolderPathW. This is a deprecated function and doesn't support
"Downloads" folders.
As a replacement, this commit uses the newer SHGetKnownFolderPath
function, which is supported since Windows Vista. Godot 3.0 only
supports Windows 7+, so we don't need to use SHGetFolderPathW for
backwards compatibility.
Fixes#26876
(cherry picked from commit 3d908f57d8)