chore(deps): update dependency google-cloud-spanner to v3.28.0 (#905)

Author: renovate-bot

samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.27.1
+google-cloud-spanner==3.28.0
 futures==3.4.0; python_version < "3"