Update google-cloud-pubsub requirement (#2168)

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...v2.25.2)

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

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requirements/extras/gcpubsub.txt

@@ -1,3 +1,3 @@
-google-cloud-pubsub>=2.18.4,<=2.20.3
+google-cloud-pubsub>=2.18.4,<=2.25.2
 google-cloud-monitoring>=2.16.0
 grpcio==1.67.0