Bump FluentAssertions from 6.8.0 to 6.9.0

Bumps [FluentAssertions](https://github.com/fluentassertions/fluentassertions) from 6.8.0 to 6.9.0.
- [Release notes](https://github.com/fluentassertions/fluentassertions/releases)
- [Changelog](https://github.com/fluentassertions/fluentassertions/blob/develop/AcceptApiChanges.ps1)
- [Commits](fluentassertions/fluentassertions@6.8.0...6.9.0)

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

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

Author: dependabot[bot]

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -8,7 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.msbuild" Version="3.2.0" PrivateAssets="all" />
-    <PackageReference Include="FluentAssertions" Version="6.8.0" />
+    <PackageReference Include="FluentAssertions" Version="6.9.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
     <PackageReference Include="PublicApiGenerator" Version="10.3.0" />
     <PackageReference Include="ReportGenerator" Version="5.1.13" PrivateAssets="all" />