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

[sdojjy]

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The DeleteObjects API is not supported in the GCS XML API. That will cause the "NotImplemented" error if the caller use "s3://" schema instead of "gcs://"
see pingcap/tidb#50918

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[benmeadowcroft]

Looks good. My main suggestion is to expand the GCS abbreviation so it is clear that it is referring to Google Cloud Storage.

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[kennytm]

Suggested change

	> Please note that the XML API of Google Cloud Storage (GCS) is not fully compatible with the s3:// protocol. If you are using GCS, please use the gcs:// protocol instead.
	> Please note that the XML API of Google Cloud Storage (GCS) is not fully compatible with the `s3://` protocol. If you are using GCS, please use the `gcs://` protocol instead.