build(deps): bump FluentAssertions from 7.1.0 to 8.1.1

Bumps [FluentAssertions](https://github.com/fluentassertions/fluentassertions) from 7.1.0 to 8.1.1.
- [Release notes](https://github.com/fluentassertions/fluentassertions/releases)
- [Changelog](https://github.com/fluentassertions/fluentassertions/blob/main/AcceptApiChanges.ps1)
- [Commits](fluentassertions/fluentassertions@7.1.0...8.1.1)

---
updated-dependencies:
- dependency-name: FluentAssertions
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

test/Directory.Packages.props

@@ -3,7 +3,7 @@
   <Import Project="$([MSBuild]::GetPathOfFileAbove('$(MSBuildThisFile)', '$(MSBuildThisFileDirectory)../'))" />
 
   <ItemGroup>
-    <PackageVersion Include="FluentAssertions" Version="[7.1.0]" />
+    <PackageVersion Include="FluentAssertions" Version="[8.1.1]" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
     <PackageVersion Include="Verify.DiffPlex" Version="3.1.2" />
     <PackageVersion Include="Verify.Xunit" Version="28.13.0" />