Bump TUnit from 1.21.6 to 1.21.20

---
updated-dependencies:
- dependency-name: TUnit
  dependency-version: 1.21.20
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/Directory.Packages.props

@@ -18,6 +18,6 @@
     <PackageVersion Include="Verify.ImageMagick" Version="3.8.6" />
     <PackageVersion Include="Verify.Serilog" Version="3.2.0" />
     <PackageVersion Include="Verify.TUnit" Version="31.13.5" />
-    <PackageVersion Include="TUnit" Version="1.21.6" />
+    <PackageVersion Include="TUnit" Version="1.21.20" />
   </ItemGroup>
 </Project>