MarkdownTextBlock not working in multithreaded window

### Describe the bug

`MarkdownTextBlock` not working in multithreaded window both in UWP and WAS.

```shell
System.Runtime.InteropServices.COMException (0x8001010E)
at WinRT.ExceptionHelpers.g__Throw|38_0(Int32 hr)
at ABI.Windows.UI.Xaml.Documents.ITextElementMethods.set_Foreground(IObjectReference _obj, Brush value)
at Windows.UI.Xaml.Documents.TextElement.set_Foreground(Brush value)
at CommunityToolkit.WinUI.Controls.TextElements.MyHeading.SetHProperties(Int32 level)
at CommunityToolkit.WinUI.Controls.TextElements.MyHeading..ctor(HeadingBlock headingBlock, MarkdownConfig config)
at CommunityToolkit.WinUI.Controls.Renderers.ObjectRenderers.HeadingRenderer.Write(WinUIRenderer renderer, HeadingBlock obj)
at Markdig.Renderers.MarkdownObjectRenderer`2.Write(RendererBase renderer, MarkdownObject obj)
at Markdig.Renderers.RendererBase.Write(MarkdownObject obj)
at Markdig.Renderers.RendererBase.WriteChildren(ContainerBlock containerBlock)
at Markdig.Renderers.RendererBase.Write(MarkdownObject obj)
at CommunityToolkit.WinUI.Controls.Renderers.WinUIRenderer.Render(MarkdownObject markdownObject)
at CommunityToolkit.WinUI.Controls.MarkdownTextBlock.ApplyText(Boolean rerender)
at CommunityToolkit.WinUI.Controls.MarkdownTextBlock.Build()
at CommunityToolkit.WinUI.Controls.MarkdownTextBlock.OnApplyTemplate()
at Windows.UI.Xaml.FrameworkElement.Windows.UI.Xaml.IFrameworkElementOverrides.OnApplyTemplate()
at ABI.Windows.UI.Xaml.IFrameworkElementOverrides.Do_Abi_OnApplyTemplate_2(IntPtr thisPtr)
```

### Steps to reproduce

```text
1. Use it such as [UWP](https://github.com/wherewhere/CoreAppUWP/blob/wuxc/CoreAppUWP/Pages/SettingsPages/SettingsPage.xaml#L395-L402) or [WAS](https://github.com/wherewhere/CoreAppUWP/blob/muxc/CoreAppUWP/Pages/SettingsPages/SettingsPage.xaml#L419-L426)
2. Run it and open two different thread windows.
3. It broken.
```

### Expected behavior

Works fine.

### Screenshots


### Code Platform

- [x] UWP
- [x] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [x] Other (specify)

### Other Windows Build number

26200.7623

### App minimum and target SDK version

- [x] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

Preview

### Visual Studio Build Number

Insiders [11408.92]

### Device form factor

Desktop

### Additional context

This problem is exits since the control is here. I don't know if here already has a issue.

### Help us help you

Yes, I'd like to be assigned to work on this item.