chore(spanner): bump google-cloud-spanner to 3.71.0 in samples (#18284)

Update samples/samples/requirements.txt to google-cloud-spanner==3.71.0
to include Cloud Spanner Queues mutation API (Batch.send and Batch.ack).

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Fixes #<issue_number_goes_here> 🦕

Author: sakthivelmanii

packages/google-cloud-spanner/samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.66.0
+google-cloud-spanner==3.71.0
 futures==3.4.0; python_version < "3"