Bump xunit from 2.8.1 to 2.9.0 (#613)

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
...

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

Author: dependabot[bot]

BitFaster.Caching.UnitTests.Std/BitFaster.Caching.UnitTests.Std.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="Moq" Version="4.20.70" />
     <PackageReference Include="ObjectLayoutInspector" Version="0.1.4" />
-    <PackageReference Include="xunit" Version="2.8.1" />
+    <PackageReference Include="xunit" Version="2.9.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>

BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="Moq" Version="4.20.69" />
     <PackageReference Include="ObjectLayoutInspector" Version="0.1.4" />
-    <PackageReference Include="xunit" Version="2.8.1" />
+    <PackageReference Include="xunit" Version="2.9.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>