Bump FluentAssertions from 6.11.0 to 6.12.0 (#510)

Bumps [FluentAssertions](https://github.com/fluentassertions/fluentassertions) from 6.11.0 to 6.12.0.
- [Release notes](https://github.com/fluentassertions/fluentassertions/releases)
- [Changelog](https://github.com/fluentassertions/fluentassertions/blob/develop/AcceptApiChanges.ps1)
- [Commits](fluentassertions/fluentassertions@6.11.0...6.12.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

NGitLab.Mock.Tests/NGitLab.Mock.Tests.csproj

@@ -10,6 +10,6 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.1" />
     <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
-    <PackageReference Include="FluentAssertions" Version="6.11.0" />
+    <PackageReference Include="FluentAssertions" Version="6.12.0" />
   </ItemGroup>
 </Project>