Bump xunit from 2.6.1 to 2.8.0

Bumps [xunit](https://github.com/xunit/xunit) from 2.6.1 to 2.8.0.
- [Commits](xunit/xunit@2.6.1...2.8.0)

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

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

test/NLog.Mongo.Tests/NLog.Mongo.Tests.csproj

@@ -29,7 +29,7 @@
     <PackageReference Include="Microsoft.Extensions.Logging" Version="7.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
     <PackageReference Include="Scrutor" Version="4.2.2" />
-    <PackageReference Include="xunit" Version="2.6.1" />
+    <PackageReference Include="xunit" Version="2.8.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.7">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>