Bump FluentAssertions to 8.4.0 (#121)

Author: renovate[bot]

tests/Prometheus.Client.MetricPusher.HostedService.Tests.csproj

@@ -5,7 +5,7 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="FluentAssertions" Version="8.3.0" />
+    <PackageReference Include="FluentAssertions" Version="8.4.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="8.0.1" />