build(deps): bump xunit from 2.7.0 to 2.8.1

Bumps [xunit](https://github.com/xunit/xunit) from 2.7.0 to 2.8.1.
- [Commits](xunit/xunit@2.7.0...2.8.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tests/LuhnDotNetTest.csproj

@@ -9,7 +9,7 @@
 
     <ItemGroup>
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
-        <PackageReference Include="xunit" Version="2.7.0" />
+        <PackageReference Include="xunit" Version="2.8.1" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.8.1">
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
             <PrivateAssets>all</PrivateAssets>