Deprecate `background upload` for object storage

What does this MR do and why?

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This is a late deprecation addition that is based on a recent discussion in #26600 (comment 883496262)

In short, supporting two ways of uploading files to object storage is causing maintenance overhead. Given the small amount of customers impacted, we will deprecate this and remove in 15.0

We have two months to provide guidance on migrations.

Addresses #26600 (closed)

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