Update dependency TUnit to 1.18.21

Author: renovate-bot

Directory.Packages.props

@@ -27,7 +27,7 @@
 
         <!-- Testing -->
         <PackageVersion Include="BenchmarkDotNet" Version="0.15.8"/>
-        <PackageVersion Include="TUnit" Version="1.17.0"/>
+        <PackageVersion Include="TUnit" Version="1.18.21"/>
         <PackageVersion Include="Shouldly" Version="4.3.0"/>
 
         <!-- Development Tools -->