deps(nuget): Bump FluentAssertions from 6.12.0 to 8.9.0

---
updated-dependencies:
- dependency-name: FluentAssertions
  dependency-version: 8.9.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: FluentAssertions
  dependency-version: 8.9.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: FluentAssertions
  dependency-version: 8.9.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: FluentAssertions
  dependency-version: 8.9.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

backend/tests/Taskdeck.Api.Tests/Taskdeck.Api.Tests.csproj

@@ -21,7 +21,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="FluentAssertions" Version="6.12.0" />
+    <PackageReference Include="FluentAssertions" Version="8.9.0" />
   </ItemGroup>
 
   <ItemGroup>