Add support for firebase-functions@7

### Description

Nov 10 2025 v7 of `firebase-functions` has been released: https://github.com/firebase/firebase-functions/releases/tag/v7.0.0

We actually only support v6 and need to check if v7 does work OOTB or there are some adaptions needed.

It also seems that ESM is now supported in Cloud Functions for Firebase: https://firebase.google.com/docs/functions/manage-functions#choose-module-system

## Things to check

- [ ] Check if we can support it OOTB
- [ ] See if we can set a `--import` label and support ESM
- if it doesn't work, we need to add this as a limitation in our docs
- if it does work, we need to tell people how to set this up properly in the docs