RootFolder property doesn't work for modernize ForderBrowserDialog

* .NET Core Version:
.Net Core 5.0 from master branch: 5.0.100-preview.5.20262.7

* Have you experienced this same bug with .NET Framework?: No

**Minimal repro:**
1. Create a Winforms .Net Core Application
2. Add following code to show modernize folderBrowserDialog and set its RootFolder property to specific Folder
```cs
public Form1()
{
InitializeComponent();
FolderBrowserDialog folderBrowserDialog = new FolderBrowserDialog();
**folderBrowserDialog.RootFolder = Environment.SpecialFolder.CommonStartMenu;**
folderBrowserDialog.ShowDialog();
}
```
3. Build and run application

**Actual behavior:**
RootFolder property setting doesn't work, please see following screenshot:

**Expected behavior:**
RootFolder should be changed in modernize folderBrowserDialog, see below screenshot in old folderBrowserDialog(.Net Framework behavior)