chore(deps): bump actions/download-artifact from 4 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](actions/download-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/download-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/msbuild.yml

@@ -113,13 +113,13 @@ jobs:
           dotnet tool run coco "Source/DafnyCore/Dafny.atg" -namespace Microsoft.Dafny -frames "Source/DafnyCore/Coco"
 
       - name: Download integration test artifacts
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v6
         with:
           pattern: integration-test-results-ubuntu-22.04-*
           merge-multiple: true
 
       - name: Download unit test artifacts
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v6
         with:
           name: unit-test-results-ubuntu-22.04