fix: use private key, not client secret

Author: gadomski

.github/workflows/release-please.yml

@@ -18,7 +18,7 @@ jobs:
         id: generate-token
         with:
           app-id: ${{ secrets.RELEASE_BOT_CLIENT_ID }}
-          private-key: ${{ secrets.RELEASE_BOT_CLIENT_SECRET }}
+          private-key: ${{ secrets.RELEASE_BOT_PRIVATE_KEY }}
       - uses: googleapis/release-please-action@v4
         with:
           token: ${{ steps.generate-token.outputs.token }}