Revert 'FluentAssertions' upgrade

The upgrade causes some tests to fail with `NullReferenceException`, `ArgumentNullException` and `ArgumentException`.

[Revertable]: Revert or Drop this patch from the tree when the above mentioned issue is fixed.

Author: Nirmal4G

Directory.Packages.props

@@ -45,7 +45,7 @@
     <PackageVersion Include="MSTest.TestFramework" Version="2.2.6" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <!-- Test Helpers -->
-    <PackageVersion Include="FluentAssertions" Version="6.1.0" />
+    <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 -->