Bump xunit in /docs/azure/sdk/snippets/unit-testing in the dotnet group (#42713)

Bumps the dotnet group in /docs/azure/sdk/snippets/unit-testing with 1 update: [xunit](https://github.com/xunit/xunit).


Updates `xunit` from 2.9.0 to 2.9.1
- [Commits](xunit/xunit@2.9.0...2.9.1)

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: dotnet
...

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

Author: dependabot[bot]

docs/azure/sdk/snippets/unit-testing/UnitTestingSampleApp.csproj

@@ -15,7 +15,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageReference Include="Moq" Version="[4.20.72]" /> <!-- Context: https://github.com/Azure/azure-sdk-for-net/issues/38111 -->
     <PackageReference Include="NSubstitute" Version="5.1.0" />
-    <PackageReference Include="xunit" Version="2.9.0" />
+    <PackageReference Include="xunit" Version="2.9.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>