Bump xunit from 2.8.0 to 2.9.0 in /src

Bumps xunit from 2.8.0 to 2.9.0.

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

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

src/BBT.StrategyPattern.Tests/BBT.StrategyPattern.Tests.csproj

@@ -27,7 +27,7 @@
     <PackageReference Include="FluentAssertions" Version="6.12.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="Ninject" Version="3.3.6" />
-    <PackageReference Include="xunit" Version="2.8.0" />
+    <PackageReference Include="xunit" Version="2.9.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.1">
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>