Bump xunit from 2.7.1 to 2.8.0 (#216)

Bumps [xunit](https://github.com/xunit/xunit) from 2.7.1 to 2.8.0.
- [Commits](xunit/xunit@2.7.1...2.8.0)

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

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

test/NLog.MailKit.Tests/NLog.MailKit.Tests.csproj

@@ -11,7 +11,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageReference Include="SmtpServer" Version="7.2.0" />
-    <PackageReference Include="xunit" Version="2.7.1" />
+    <PackageReference Include="xunit" Version="2.8.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.8">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>