Revert package upgrades

1. 'Win2D.uwp'
2. 'FluentAssertions'

These upgrades cause some tests to fail with `NullReferenceException`, `ArgumentNullException` and `ArgumentException`.

[Revertable]: Revert or Drop this patch from the tree when those issues are fixed.

Author: Nirmal4G

Directory.Packages.props

@@ -6,7 +6,7 @@
 
   <ItemGroup Label="Core">
     <!-- UWP SDK -->
-    <PackageVersion Include="Win2D.uwp" Version="1.26.0" />
+    <PackageVersion Include="Win2D.uwp" Version="1.25.0" />
     <PackageVersion Include="ColorCode.UWP" Version="2.0.12" />
     <PackageVersion Include="Microsoft.UI.Xaml" Version="2.7.1" />
     <PackageVersion Include="Microsoft.Xaml.Behaviors.Uwp.Managed" Version="2.0.1" />
@@ -49,7 +49,7 @@
     <PackageVersion Include="MSTest.TestFramework" Version="2.2.9" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
     <!-- Test Helpers -->
-    <PackageVersion Include="FluentAssertions" Version="6.6.0" />
+    <PackageVersion Include="FluentAssertions" Version="5.10.3" />
     <!-- DO NOT UPGRADE: This version is used for upgrade testing between 6.1.1 and 7.0.0 for the Object Serializers -->
     <PackageVersion Include="Newtonsoft.Json" Version="10.0.3" Pin="true" />
     <!-- WinUI Test Infra -->