Bump TUnit from 1.20.0 to 1.21.6 (#17)

---
updated-dependencies:
- dependency-name: TUnit
  dependency-version: 1.21.6
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: minor-and-patch
...

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

Author: dependabot[bot]

src/Directory.Packages.props

@@ -15,7 +15,7 @@
     <!-- Versioning -->
     <PackageVersion Include="SimpleBranchVersioning" Version="0.6.1" />
     <!-- Testing -->
-    <PackageVersion Include="TUnit" Version="1.20.0" />
+    <PackageVersion Include="TUnit" Version="1.21.6" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp.Analyzer.Testing" Version="1.1.3" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="5.3.0" />
     <PackageVersion Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.2" />