Bump protobufjs, @google-cloud/pubsub and firebase-admin in /backend

Bumps [protobufjs](https://github.com/protobufjs/protobuf.js) to 7.2.6 and updates ancestor dependencies [protobufjs](https://github.com/protobufjs/protobuf.js), [@google-cloud/pubsub](https://github.com/googleapis/nodejs-pubsub) and [firebase-admin](https://github.com/firebase/firebase-admin-node). These dependencies need to be updated together.


Updates `protobufjs` from 7.2.4 to 7.2.6
- [Release notes](https://github.com/protobufjs/protobuf.js/releases)
- [Changelog](https://github.com/protobufjs/protobuf.js/blob/master/CHANGELOG.md)
- [Commits](protobufjs/protobuf.js@protobufjs-v7.2.4...protobufjs-v7.2.6)

Updates `@google-cloud/pubsub` from 3.2.1 to 4.4.0
- [Release notes](https://github.com/googleapis/nodejs-pubsub/releases)
- [Changelog](https://github.com/googleapis/nodejs-pubsub/blob/main/CHANGELOG.md)
- [Commits](googleapis/nodejs-pubsub@v3.2.1...v4.4.0)

Updates `firebase-admin` from 11.4.1 to 12.1.0
- [Release notes](https://github.com/firebase/firebase-admin-node/releases)
- [Commits](firebase/firebase-admin-node@v11.4.1...v12.1.0)

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

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