ci: bump codecov-action from v5 to v6

Author: appleboy

.github/workflows/testing.yml

@@ -74,7 +74,7 @@ jobs:
           make test
 
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           flags: ${{ matrix.os }},go-${{ matrix.go }}
           token: ${{ secrets.CODECOV_TOKEN }}