chore(deps): update dependency google-cloud-spanner to v3.21.0 (#814)

Author: renovate-bot

samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.20.0
+google-cloud-spanner==3.21.0
 futures==3.3.0; python_version < "3"