Downgrade codecov from v4 to v3 because the tokenless upload does not work as described in the repository's README in some cases for a public repository

- codecov/codecov-action#1292

Author: KatsukiFujimoto

.github/workflows/tests.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Run tests
         run: bundle exec rspec
       - name: Upload coverage reports to Codecov
-        uses: codecov/codecov-action@v4
+        uses: codecov/codecov-action@v3
         env:
           CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}