Update Codecov action to v6

Author: kvz

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
         run: bundle exec rake test
       - name: Upload coverage to Codecov
         if: matrix.ruby == '3.3'
-        uses: codecov/codecov-action@75cd11691c0faa626561e295848008c8a7dddffe
+        uses: codecov/codecov-action@fb8b3582c8e4def4969c97caa2f19720cb33a72f
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           files: ./coverage/coverage.json