CI: update codecov/codecov-action to v6

Author: fingolfin

.github/workflows/CI.yml

@@ -43,6 +43,6 @@ jobs:
         with:
           only-needed: true
       - uses: gap-actions/process-coverage@v2
-      - uses: codecov/codecov-action@v5
+      - uses: codecov/codecov-action@v6
         with:
           token: ${{ secrets.CODECOV_TOKEN }}