⬆️ update TUnit to 1.18.21 (#38)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/Directory.Packages.props

@@ -13,7 +13,7 @@
     <PackageVersion Include="System.CommandLine" Version="2.0.3" />
 
     <!-- Test dependencies -->
-    <PackageVersion Include="TUnit" Version="1.18.9" />
+    <PackageVersion Include="TUnit" Version="1.18.21" />
     <PackageVersion Include="AwesomeAssertions" Version="9.4.0" />
   </ItemGroup>
 </Project>