use codecov action instead of deprecated npm package (DataDog#2059)

Author: rochdev

.github/workflows/core.yml

@@ -20,4 +20,4 @@ jobs:
       - run: yarn test:shimmer:ci
       - uses: ./.github/actions/node/latest
       - run: yarn test:shimmer:ci
-      - run: yarn codecov
+      - uses: codecov/codecov-action@v2