Bump protobufjs and google-gax in /big-dipper

Bumps [protobufjs](https://github.com/protobufjs/protobuf.js) and [google-gax](https://github.com/googleapis/gax-nodejs). These dependencies needed to be updated together.

Updates `protobufjs` from 7.1.2 to 7.2.4
- [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-v7.1.2...protobufjs-v7.2.4)

Updates `google-gax` from 3.5.2 to 3.6.1
- [Release notes](https://github.com/googleapis/gax-nodejs/releases)
- [Changelog](https://github.com/googleapis/gax-nodejs/blob/main/CHANGELOG.md)
- [Commits](googleapis/gax-nodejs@v3.5.2...v3.6.1)

---
updated-dependencies:
- dependency-name: protobufjs
  dependency-type: indirect
- dependency-name: google-gax
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>