Bump bunit from 1.20.8 to 1.21.9

Bumps [bunit](https://github.com/bUnit-dev/bUnit) from 1.20.8 to 1.21.9.
- [Release notes](https://github.com/bUnit-dev/bUnit/releases)
- [Changelog](https://github.com/bUnit-dev/bUnit/blob/main/CHANGELOG.md)
- [Commits](bUnit-dev/bUnit@v1.20.8...v1.21.9)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tests/Directory.Build.props

@@ -2,7 +2,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
   <ItemGroup Condition="$(MSBuildProjectName) != 'LinkDotNet.Blog.TestUtilities'">
-    <PackageReference Include="bunit" Version="1.20.8" />
+    <PackageReference Include="bunit" Version="1.21.9" />
     <PackageReference Include="FluentAssertions" Version="6.11.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.3" />
     <PackageReference Include="Moq" Version="4.18.4" />