Bump the xunit group with 2 updates

Bumps the xunit group with 2 updates: [xunit](https://github.com/xunit/xunit) and [xunit.runner.visualstudio](https://github.com/xunit/visualstudio.xunit).


Updates `xunit` from 2.6.3 to 2.6.4
- [Commits](xunit/xunit@2.6.3...2.6.4)

Updates `xunit.runner.visualstudio` from 2.5.5 to 2.5.6
- [Release notes](https://github.com/xunit/visualstudio.xunit/releases)
- [Commits](xunit/visualstudio.xunit@2.5.5...2.5.6)

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

Directory.Packages.props

@@ -40,7 +40,7 @@
     <PackageVersion Include="System.Threading.RateLimiting" Version="$(MicrosoftExtensionsVersion)" />
     <PackageVersion Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
     <PackageVersion Include="System.ValueTuple" Version="4.5.0" />
-    <PackageVersion Include="xunit" Version="2.6.3" />
-    <PackageVersion Include="xunit.runner.visualstudio" Version="2.5.5" />
+    <PackageVersion Include="xunit" Version="2.6.4" />
+    <PackageVersion Include="xunit.runner.visualstudio" Version="2.5.6" />
   </ItemGroup>
 </Project>