chore(deps): update dependency tunit to 1.21.6

Author: renovate[bot]

src/Directory.Packages.props

@@ -30,7 +30,7 @@
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="6.1.0" />
     <PackageVersion Include="Microsoft.Testing.Extensions.CodeCoverage" Version="18.5.2" />
     <PackageVersion Include="PublicApiGenerator" Version="11.5.4" />
-    <PackageVersion Include="TUnit" Version="1.19.74" />
+    <PackageVersion Include="TUnit" Version="1.21.6" />
     <PackageVersion Include="Verify.TUnit" Version="31.13.5" />
   </ItemGroup>
 </Project>