Bump firebase-admin from 11.5.0 to 12.0.0 in /functions

Bumps [firebase-admin](https://github.com/firebase/firebase-admin-node) from 11.5.0 to 12.0.0.
- [Release notes](https://github.com/firebase/firebase-admin-node/releases)
- [Commits](firebase/firebase-admin-node@v11.5.0...v12.0.0)

---
updated-dependencies:
- dependency-name: firebase-admin
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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