chore(deps): update dependency fluentassertions to v8

Author: renovate[bot]

src/ReactiveMvvm.Tests/ReactiveMvvm.Tests.csproj

@@ -10,7 +10,7 @@
         <PrivateAssets>all</PrivateAssets>
         <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="FluentAssertions" Version="6.12.0" />
+    <PackageReference Include="FluentAssertions" Version="8.2.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="NSubstitute" Version="5.1.0" />
     <PackageReference Include="ReactiveUI.Testing" Version="20.2.45" />