[DOCS] for sample: /quickstarts/email-users

### Which sample?

[Firebase SDK for Cloud Functions Quickstart - Auth triggers](https://github.com/firebase/functions-samples/tree/main/quickstarts/email-users)

### What is the issue with this sample's docs?

[Outdated step 8.](https://github.com/firebase/functions-samples/tree/main/quickstarts/email-users#setting-up-the-sample)

Unfortunatly the [Less Secure Apps](https://www.google.com/settings/security/lesssecureapps) doesn't seem [available anymore](https://support.google.com/accounts/answer/6010255?authuser=3&hl=en&authuser=3&visit_id=638035104309275663-341291806&p=less-secure-apps&rd=1#zippy=)