Bump google-cloud-pubsub from 2.18.1 to 2.18.4 (#185)

Bumps [google-cloud-pubsub](https://github.com/googleapis/python-pubsub) from 2.18.1 to 2.18.4.
- [Release notes](https://github.com/googleapis/python-pubsub/releases)
- [Changelog](https://github.com/googleapis/python-pubsub/blob/main/CHANGELOG.md)
- [Commits](googleapis/python-pubsub@v2.18.1...v2.18.4)

---
updated-dependencies:
- dependency-name: google-cloud-pubsub
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>