Bump google-cloud-pubsub from 2.18.4 to 2.21.5

Bumps [google-cloud-pubsub](https://github.com/googleapis/python-pubsub) from 2.18.4 to 2.21.5.
- [Release notes](https://github.com/googleapis/python-pubsub/releases)
- [Changelog](https://github.com/googleapis/python-pubsub/blob/main/CHANGELOG.md)
- [Commits](googleapis/python-pubsub@v2.18.4...v2.21.5)

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

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

requirements.txt

@@ -4,6 +4,6 @@ lxml==4.9.3
 numpy==1.26.1
 fastavro==1.8.4
 six>=1.12.0
-google-cloud-pubsub==2.18.4
+google-cloud-pubsub==2.21.5
 google-cloud-bigquery==3.12.0
 pyyaml==6.0.1