Use actions/checkout v2 for heroku-deploy workflow as written in README

Author: yasulab

.github/workflows/test.yml

@@ -53,7 +53,7 @@ jobs:
     if: github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v2
       - uses: akhileshns/[email protected]
         with:
           heroku_api_key:  ${{ secrets.HEROKU_API_KEY }}