Downgrade FluentAssertions to 5.10.3 for compatibility with .NET Framework net452.

Author: TimSirmovics

test/Directory.Build.props

@@ -12,7 +12,7 @@
     <PackageReference Include="NUnit" Version="3.13.*"/>
     <PackageReference Include="NUnit3TestAdapter" Version="4.0.0"/>
     <PackageReference Include="AutoFixture" Version="4.17.0"/>
-    <PackageReference Include="FluentAssertions" Version="6.2.0"/>
+    <PackageReference Include="FluentAssertions" Version="5.10.3"/>
     <PackageReference Include="coverlet.collector" Version="3.1.0"/>
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All" />
   </ItemGroup>