Bump FluentAssertions to 8.6.0 (#105)

Author: renovate[bot]

tests/Prometheus.Client.HttpClient.Tests/Prometheus.Client.HttpClient.Tests.csproj

@@ -6,7 +6,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Fennel" Version="0.3.0" />
-    <PackageReference Include="FluentAssertions" Version="8.5.0" />
+    <PackageReference Include="FluentAssertions" Version="8.6.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="xunit" Version="2.9.3" />