Updated System.Security.Cryptography.Xml to v4.7.1 due to security vulnerability (#21672)

Author: vidai-msft

tools/TestFx/TestFx.csproj

@@ -35,6 +35,7 @@
 	<PackageReference Include="Moq" Version="4.7.145" />
 	<PackageReference Include="System.Data.SqlClient" Version="4.8.5" />
 	<PackageReference Include="System.DirectoryServices.Protocols" Version="5.0.1" />
+	<PackageReference Include="System.Security.Cryptography.Xml" Version="4.7.1" />
   </ItemGroup>
 
 </Project>