Update export-backups-to-own-cloud-account.md

[aashishkohli]

Updates:

1. Clarified that cross-region backup/restores are supported but cross-cloud is currently not supported.
2. Added AWS role based auth details to the AWS section as an alternative to key/secret based auth.

Summary

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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