Known issue: WPF Image memory leak when remove image from visual tree

* .NET Core Version: 3.1.100
* Windows version: 18363
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
* Is this bug related specifically to tooling in Visual Studio No

**Problem description:**

The image source will memory leak when we remove the image from visual tree before we set the image source as null

Step:

1. Set the image source
2. Add image in the VisualTree and wait the image show
3. Remove image from the visual tree and then set the image source as null

And you can find the image source object never free

```
private void Button_Click(object sender, RoutedEventArgs e)
{
// Remove the current Image control from the visual tree and set source is null when click button.
// Then new a image control and add source to the RenderTargetBitmap object and show it.
// You can see the gc never delete the RenderTargetBitmap object that make memory leak.

var oldBorder = RootGrid.Children.OfType().LastOrDefault();
if (oldBorder != null)
{
var oldImage = (Image)oldBorder.Child;

// In order to solve it , you should set the image.Source is null and use UpdateLayout.
// The below code can solve it.
// oldImage.Source = null;
// oldImage.UpdateLayout();

// Remove the current Image control from the visual tree.
RootGrid.Children.Remove(oldBorder);
oldImage.Source = null;
Borders.Add(oldBorder);
}

var bitmap = new RenderTargetBitmap(1024, 1024, 96, 96, PixelFormats.Default);

var image = new Image { Source = bitmap };
var border = new Border { Child = image };
RootGrid.Children.Add(border);

// In order to facilitate changes in memory, after each operation will be garbage collection
GC.Collect();
}

public readonly List Borders = new List();
```

**Actual behavior:**

The Image memory leak

**Expected behavior:**

The image source object can be free

**Minimal repro:**

https://github.com/dotnet-campus/wpf-issues/tree/master/ImageMemoryLeakDotNetCore