Bump cloud.google.com/go/pubsub from 1.20.0 to 1.21.0

Bumps [cloud.google.com/go/pubsub](https://github.com/googleapis/google-cloud-go) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/googleapis/google-cloud-go/releases)
- [Changelog](https://github.com/googleapis/google-cloud-go/blob/main/CHANGES.md)
- [Commits](https://github.com/googleapis/google-cloud-go/compare/pubsub/v1.20.0...pubsub/v1.21.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -4,7 +4,7 @@ go 1.18
 
 require (
 	cloud.google.com/go v0.100.2 // indirect
-	cloud.google.com/go/pubsub v1.20.0
+	cloud.google.com/go/pubsub v1.21.0
 	cloud.google.com/go/storage v1.22.0
 	github.com/Azure/azure-pipeline-go v0.2.3
 	github.com/Azure/azure-storage-blob-go v0.14.0