ci: update release workflow to use secrets for app ID

Author: amondnet

.github/workflows/release-please.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/create-github-app-token@v2
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
+          app-id: ${{ secrets.APP_ID }}
           private-key: ${{ secrets.PRIVATE_KEY }}
       - uses: googleapis/release-please-action@v4
         with: