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

Author: renovate[bot]

.github/workflows/GnuTests.yml

@@ -111,7 +111,7 @@ 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