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

[aashishkohli]

Updating docs to say Azure BYOB is not available for now due to this issue: https://clickhouse-inc.slack.com/archives/C02H133R3LK/p1738105361466469

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/en/integrations/index.mdx