Add information about backup region for BYOK in GitLab Dedicated

What does this MR do and why?

Adds more information about AWS KMS keys and the region they need to be available in when a customer chooses to use BYOK in GitLab Dedicated.

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.