chore(deps): update dependency protobuf to v3.18.1 (#324)

Author: renovate-bot

bigquery_storage/snippets/requirements.txt

@@ -1,3 +1,3 @@
 google-cloud-bigquery-storage==2.9.0
 google-cloud-bigquery==2.28.0
-protobuf==3.18.0
+protobuf==3.18.1