docs: 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 possible violations 😱

There is a possible violation for removing region tag in use.

• samples/snippets/subscriber.py:194, tag pubsub_create_push_no_wrapper_subscription(usage: page 1).

The end of the violation section. All the stuff below is FYI purposes only.

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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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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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