Bump TUnit.Core from 1.19.57 to 1.21.6

---
updated-dependencies:
- dependency-name: TUnit.Core
  dependency-version: 1.21.6
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

Directory.Packages.props

@@ -20,6 +20,6 @@
     <!-- Reference-only packages for test framework namespace resolution in analyzer tests -->
     <PackageVersion Include="xunit.v3.core" Version="3.2.2" />
     <PackageVersion Include="NUnit" Version="4.5.1" />
-    <PackageVersion Include="TUnit.Core" Version="1.19.57" />
+    <PackageVersion Include="TUnit.Core" Version="1.21.6" />
   </ItemGroup>
 </Project>