Update dependency google-cloud-pubsub to v1.4.2 [(#3340)](GoogleCloudPlatform/python-docs-samples#3340)

Co-authored-by: Leah E. Cole <[email protected]>

Author: busunkim96

samples/snippets/requirements.txt

@@ -1,5 +1,5 @@
 google-cloud-dlp==0.14.0
 google-cloud-storage==1.26.0
-google-cloud-pubsub==1.3.1
+google-cloud-pubsub==1.4.2
 google-cloud-datastore==1.12.0
 google-cloud-bigquery==1.24.0