Bump xunit from 2.5.3 to 2.8.1 (#401)

Bumps [xunit](https://github.com/xunit/xunit) from 2.5.3 to 2.8.1.
- [Commits](xunit/xunit@2.5.3...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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Directory.Packages.props

@@ -19,7 +19,7 @@
     <PackageVersion Include="EntityFramework" Version="6.4.4" />
     <PackageVersion Include="Akka.Hosting.TestKit" Version="$(AkkaVersion)" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
-    <PackageVersion Include="xunit" Version="2.5.3" />
+    <PackageVersion Include="xunit" Version="2.8.1" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.8.1" />
     <PackageVersion Include="Akka.TestKit.Xunit2" Version="$(AkkaVersion)" />
     <PackageVersion Include="Akka.Streams.TestKit" Version="$(AkkaVersion)" />