Update google-cloud-pubsub requirement

Updates the requirements on [google-cloud-pubsub](https://github.com/googleapis/python-pubsub) to permit the latest version.
- [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...v2.26.1)

---
updated-dependencies:
- dependency-name: google-cloud-pubsub
  dependency-type: direct:production
...

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

requirements/extras/gcpubsub.txt

@@ -1,3 +1,3 @@
-google-cloud-pubsub>=2.18.4,<=2.20.3
+google-cloud-pubsub>=2.18.4,<=2.26.1
 google-cloud-monitoring>=2.16.0
 grpcio==1.67.0