ci: bump actions/upload-artifact from 4 to 6 (strands-agents#1608)

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Author: dependabot[bot]

.github/workflows/integration-test.yml

@@ -61,7 +61,7 @@ jobs:
 
       - name: Upload test results
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: test-results
           path: ./build/test-results.xml