Bump FluentAssertions from 6.2.0 to 6.4.0 (#132)

Bumps [FluentAssertions](https://github.com/fluentassertions/fluentassertions) from 6.2.0 to 6.4.0.
- [Release notes](https://github.com/fluentassertions/fluentassertions/releases)
- [Changelog](https://github.com/fluentassertions/fluentassertions/blob/master/AcceptApiChanges.ps1)
- [Commits](fluentassertions/fluentassertions@6.2.0...6.4.0)

---
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]

src/Directory.Build.props

@@ -11,7 +11,7 @@
   </PropertyGroup>
 
     <ItemGroup Condition="$(MSBuildProjectName.EndsWith('Tests'))">
-    <PackageReference Include="FluentAssertions" Version="6.2.0" />
+    <PackageReference Include="FluentAssertions" Version="6.4.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.0.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3">