Bump xunit from 2.8.1 to 2.9.0

Bumps xunit from 2.8.1 to 2.9.0.

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

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

Directory.Packages.props

@@ -8,7 +8,7 @@
         <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
         <PackageVersion Include="Sprache" Version="2.3.1" />
         <PackageVersion Include="Superpower" Version="3.0.0" />
-        <PackageVersion Include="xunit" Version="2.8.1" />
+        <PackageVersion Include="xunit" Version="2.9.0" />
         <PackageVersion Include="xunit.runner.visualstudio" Version="2.8.1" />
     </ItemGroup>