update to FluentAssertions 6.1 (#1279)

Author: SimonCropp

test/Directory.Build.props

@@ -22,7 +22,7 @@
     <PackageReference Include="coverlet.msbuild" Version="3.0.3" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <PackageReference Include="NSubstitute" Version="4.2.2" />
-    <PackageReference Include="FluentAssertions" Version="5.10.3" />
+    <PackageReference Include="FluentAssertions" Version="6.1.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
     <PackageReference Include="Verify.Xunit" Version="13.3.0" />