chore(deps): update dependency tunit to 1.21.6

Author: renovate[bot]

src/Directory.Packages.props

@@ -13,7 +13,7 @@
   <ItemGroup>
     <!-- Testing Framework -->
     <PackageVersion Include="NSubstitute" Version="5.3.0" />
-    <PackageVersion Include="TUnit" Version="1.19.22" />
+    <PackageVersion Include="TUnit" Version="1.21.6" />
     <PackageVersion Include="Verify.TUnit" Version="31.13.2" />
     <PackageVersion Include="Verify.SourceGenerators" Version="2.5.0" />