CI: Update to GHA's codecov-action@v4

Author: amotl

.github/workflows/tests.yml

@@ -84,6 +84,8 @@ jobs:
 
       # https://github.com/codecov/codecov-action
       - name: Upload coverage results to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
         with:
           fail_ci_if_error: true