Clarify separate Object Storage buckets in docs

What does this MR do?

Relates gitlab-org/quality/reference-architectures#5 (closed)

MR updates an outdated section of the Object Storage docs regarding separate buckets. While it was the case previously that separate buckets was preferred this is no longer the case with consolidated storage.

The section is now updated to call out that a combined bucket is fine but that separate buckets will be required still when Helm is used.

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