Update akhileshns/heroku-deploy action to v3.15.15 (#468)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/heroku-deploy.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: akhileshns/heroku-deploy@v3.14.15
+      - uses: akhileshns/heroku-deploy@v3.15.15
         with:
           heroku_api_key: ${{secrets.HEROKU_API_KEY}}
           heroku_app_name: ${{secrets.HEROKU_APP_NAME}}