Update dependency google-auth to v1.21.0 [(#4588)](GoogleCloudPlatform/python-docs-samples#4588)

Author: renovate-bot

samples/snippets/api/requirements.txt

@@ -1,3 +1,3 @@
 google-api-python-client==1.10.1
-google-auth==1.20.1
+google-auth==1.21.0
 google-auth-httplib2==0.0.4