Update dependency FluentAssertions to v6.8.0

Author: renovate[bot]

Directory.Packages.props

@@ -39,7 +39,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.4.5" />
     <PackageVersion Include="xunit" Version="2.4.2" />
-    <PackageVersion Include="FluentAssertions" Version="6.6.0" />
+    <PackageVersion Include="FluentAssertions" Version="6.8.0" />
     <PackageVersion Include="NSubstitute" Version="4.3.0" />
     <PackageVersion Include="Serilog.Extensions.Logging" Version="3.1.0" />
     <PackageVersion Include="Serilog.Sinks.Observable" Version="2.0.2" />