Bump codecov action, add token

Author: brndnmtthws

.github/workflows/coverage.yml

@@ -23,6 +23,7 @@ jobs:
           toolchain: nightly
       - run: cargo tarpaulin --features serde,nightly --out Xml
       - name: Upload to codecov.io
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
           fail_ci_if_error: true
+          token: ${{ secrets.CODECOV_TOKEN }}