Move About.txt info to the ThirdPartyNotice file instead.

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Markdown/About.txt

@@ -9,6 +9,7 @@ This project incorporates components from the projects listed below. The origina
 2.	MichaeIDietrich/UwpNotificationNetCoreTest commit 5c1a4a3 (https://github.com/MichaeIDietrich/UwpNotificationNetCoreTest), used in DesktopNotificationManagerCompat.cs to support .NET Core 3.0.
 3.	lbugnion/mvvmlight commit 4cbf77c (https://github.com/lbugnion/mvvmlight), from which some APIs from the `Microsoft.Toolkit.Mvvm` package take inspiration from.
 4.  PrivateObject/PrivateType (https://github.com/microsoft/testfx/tree/664ac7c2ac9dbfbee9d2a0ef560cfd72449dfe34/src/TestFramework/Extension.Desktop), included in UnitTests.
+5.  QuinnDamerell/UniversalMarkdown (https://github.com/QuinnDamerell/UniversalMarkdown) contributed by Quinn Damerell and Paul Bartrum for the MarkdownTextBlock control, relicensed to this .NET Foundation project under the MIT license upon contribution in https://github.com/windows-toolkit/WindowsCommunityToolkit/pull/772.
 
 %% PedroLamas/DeferredEvents NOTICES AND INFORMATION BEGIN HERE
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