Bump @google-cloud/firestore and firebase-admin in /functions

Bumps [@google-cloud/firestore](https://github.com/googleapis/nodejs-firestore) to 6.8.0 and updates ancestor dependency [firebase-admin](https://github.com/firebase/firebase-admin-node). These dependencies need to be updated together.


Updates `@google-cloud/firestore` from 3.8.6 to 6.8.0
- [Release notes](https://github.com/googleapis/nodejs-firestore/releases)
- [Changelog](https://github.com/googleapis/nodejs-firestore/blob/main/CHANGELOG.md)
- [Commits](googleapis/nodejs-firestore@v3.8.6...v6.8.0)

Updates `firebase-admin` from 8.13.0 to 11.11.1
- [Release notes](https://github.com/firebase/firebase-admin-node/releases)
- [Commits](firebase/firebase-admin-node@v8.13.0...v11.11.1)

---
updated-dependencies:
- dependency-name: "@google-cloud/firestore"
  dependency-type: indirect
- dependency-name: firebase-admin
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>