chore(deps): update dawidd6/action-download-artifact action to v21

Author: renovate[bot]

.github/workflows/GnuTests.yml

@@ -100,7 +100,7 @@ jobs:
           persist-credentials: false
 
       - name: Retrieve reference artifacts
-        uses: dawidd6/action-download-artifact@v20
+        uses: dawidd6/action-download-artifact@v21
         continue-on-error: true
         with:
           workflow: GnuTests.yml