Bump protobufjs in /firestore-activity-feeds/functions

Bumps [protobufjs](https://github.com/protobufjs/protobuf.js) from 7.5.4 to 7.6.1.
- [Release notes](https://github.com/protobufjs/protobuf.js/releases)
- [Changelog](https://github.com/protobufjs/protobuf.js/blob/protobufjs-v7.6.1/CHANGELOG.md)
- [Commits](protobufjs/protobuf.js@protobufjs-v7.5.4...protobufjs-v7.6.1)

---
updated-dependencies:
- dependency-name: protobufjs
  dependency-version: 7.6.1
  dependency-type: indirect
...

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Author: dependabot[bot]