Update google-cloud-storage requirement in /lambda/backup-verification

Updates the requirements on [google-cloud-storage](https://github.com/googleapis/google-cloud-python) to permit the latest version.
- [Release notes](https://github.com/googleapis/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-documentai/CHANGELOG.md)
- [Commits](https://github.com/googleapis/google-cloud-python/compare/google-cloud-storage-v3.11.0...google-cloud-storage-v3.12.0)

---
updated-dependencies:
- dependency-name: google-cloud-storage
  dependency-version: 3.12.0
  dependency-type: direct:production
...

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lambda/backup-verification/requirements.txt

@@ -1 +1 @@
-google-cloud-storage>=3.11.0,<4.0.0
+google-cloud-storage>=3.12.0,<4.0.0