`--token` is deprecated

When using the snippet I get a warning
```
Authenticating with `--token` is deprecated and will be removed in a future major version of `firebase-tools`. Instead, use a service account key with `GOOGLE_APPLICATION_CREDENTIALS`: https://cloud.google.com/docs/authentication/getting-started
```
at this line: https://github.com/firebase/snippets-node/blob/ea3747eee572d3e1bebd1cdafda9fa34a6dc38f0/firestore/solution-deletes/functions/index.js#L67

Is that because I am using it in a wrong way or is the snipped outdated? How would I use the described service account key in the case of this snippet? I appreciate your help, I am still new to GCP!!

Environment:
firebase-tools: 11.20.0
Node: 16

Steps to reproduce:
use the snipped as described

Expected behaviour:
Execute recursive delete of `path`

Actual behaviour:
Same, but with deprecation warning