use gcs:// instead of s3:// when the external storage is Google Cloud Storage

[sdojjy]

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here is https://github.com/pingcap/docs/pull/16893/files

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• This PR is translated from: use gcs:// instead of s3:// when the external storage is Google Cloud Storage docs#16893
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@sdojjy: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	6cbe22d	link	true	/test pull-verify

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[lance6716]

还是推荐 PM 检查 gcs:// 是否能完全覆盖 s3:// 的场景，用户当初想使用 s3://，现在我们想让他改成 gcs:// 会不会遇到什么困难