doc: Update CPS to GCS and push payload unwrapping sample region tag

[liuyunnnn]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/subscriber.py:194, tag pubsub_create_unwrapped_push_subscription
• samples/snippets/subscriber.py:361, tag pubsub_create_cloud_storage_subscription

You are about to delete 2 region tags.

• samples/snippets/subscriber.py:194, tag pubsub_create_push_no_wrapper_subscription
• samples/snippets/subscriber.py:361, tag pubsub_cloudstorage_subscription

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[engelke]

LGTM